These are legitimate questions, HD. The short answer is: for many reasons. One of them is the fact that after the 4th century, the West was linguistically isolated from the East; the Latins no longer understood Greek, and the reeks never bothered to learn Latin. It was pagan Rome, after all, that imitated Greeks in art, religion, and every aspect of culture and society and not the other way around.
The isolation resulted in mutual "black-out" and the fact that the particular Churches (Latin and Greek) remain for all practical purposes one Chuch, so close in everything despite millennial separation, attests to the validity of Tradition (not traditions!) that resists mutations.
When you strip linguistic, cultural, ecclesiastical, and doctrinal variations, even the half-dozen mutually exclusive ones, you find one and the same Faith and one and the same catholic and Apostolic Church. That is pretty awesome, HD! That makes us exceedingly glad and confident that the Church never swayed, despite human errors, individual corruption, politics, linguistic barriers, cultural diversions, as well as external expressions of the praxis.
St. Augustine's teachings did not cross the linguistic barrier into the East until the 15th or 16th century. The Augustinian teaching of the "original sin" was soemthing unknown to the East, which means that it was unknown to the undivided Church until that time (4th century) or else there would have been talk about it. It did not become controversial because it was never brought before an Ecumenical Council. It remained the doctrine of the West, unknown to the Greeks.
The Greeks had no reason to postulate Immaculate Conception because they never postulated St. Augustine's invention of the "original sin." It is not that the Orthodox deny that our ancestral parents sinned and fell from God's grace, but that we are not born in sin, but simply with a propensity to sin.
A little consultation with our learned Latin brothers and sisters reveals to us that they, too, believe that we are born with propensity to sin, and do not inherit the sin (and the guilt) of our ancestral parents, and that St. St. Augustine's terminology is actually misleading! Bingo!
From this knowledge, our understanding of Baptism shows convergence as well: baptism enables to resist that propensity (but doesn't remove it). It "detoxifies" us, so that when we come to, we will not be hung over but lucid and able to respond to God's call and cleave to Him. So, when our Latins brothers say Baptism washes away the "original sin," we now understand what they mean and there is no clash; and they understand that when we put the "original sin" in quotes we don't mean to call their doctrine heresy but are simply consistent with the Orthodox teaching that does not call our propensity to sin by that name.
Likewise, in the case of the Filioque, the Latin formulation is in every way true, namely that the Father gives His Spirit to His Son, and the Son to the Father, that the Spirit is the eternal eros (longing) that is eternally shared by the Father and the Son, but where the Filioque "fails" is that it is that it is incomplete, because it fails to establish that the Spirit, as regards to His existence, proceeds from the Father, to paraphrase a Palamite definition.
The issue of the Filioque surfaced more than 300 years after it was introduced at the Council of Toledo (a local council in Spain), in response to Arian heresy of the newly arrived Visigoths in the 6th century.
It was actually Frankish pilgrims in the Middle East who reported that the Greeks were reciting "romp" Creed, having "removed" the "and from the Son," from it that started the whole thin in the 9th century.
The arrival of Frankish clergy to Bulgaria (king there was "shopping" for a Church that would crown him a tsar) who noticed that the Greek clergy had long beards, were married, and were reciting the Creed in a "heretical" way, having "removed" the filioque from it.
The prompted the famous two Photian councils which would have qualified as the 8th Ecumenical Council and of which the first the Roman Catholic Church subsequently accepted as the "Eight," having renegged on the earlier acceptance of th second and reversal of the first so-called Photian Councils.
So, as you can see, the communication was not exactly "digital" in those days, and much time would pass and many formulations would form before anyone noticed what the other side was doing, and in most cases the heretofore unknown would be termed a "heresy."
Thus, there is simply no simple way to answer your question. History of the Church sheds a lot of light on what happened and how it happened, and what it shows is that nothing of what transpired was simple or easy. It has always been and still is a struggle to remain on the true path.
Unless I misunderstand what you mean by the "one and the same Faith" I would respectfully disagree. With regards to the different view between Catholics and Orthodox on the Immaculate Conception (as one example) both are saying they are following tradition set down by the fathers. Yet, both hold different views. I'm sure the Catholics wouldn't minimize the Immaculate Conception. Their whole theology is practically built around it. Yet, the Orthodox do not believe in the Immaculate Conception and they say they're following tradition. Someone has to be wrong.
The isolation you rightfully suggest shows different doctrines based upon "tradition". How can that possibly be if God leads "one and the same catholic and Apostolic Church"? Moreover the Catholic website states that the Immaculate Conception was the Church's tradition for 1200 years. That would imply that the Orthodox have strayed from sound teachings.
Protestants here are constantly told by the Orthodox and Catholics that their doctrine is based upon tradition set down by the Church and these have always been part of the Church. We are chastised for NOT following tradition. The simple fact is that both groups review and develop doctrine and then find the writings that support that doctrine and reject the writings that don't. To be fair, the Orthodox makes no pretense of following this approach as I posted from the Orthodox website several posts back. Our Catholic friends, OTOH, would like us to believe that various RCC positions were always held by the Church. Doctrines such as the Immaculate Conceptions puts them squarely at odds with the Orthodox view and is a prime example.
I'm baffled how one can say the Orthodox and Catholic view are essentially the same and I have listed the doctrinal differences. But what is stranger is that each Church state they base their views upon the tradition of the fathers. Perhaps that was Luther error. Had he just provided a few snippets from early church fathers, it would have confused everyone for the next 500 years.