Magician's Nephew is book #1 because it goes into details of the creation of Narnia, much like the Boook of Genesis of the Bible goes into details of the creation of the Heavens and the Earth, among other things.
This should indicate to you that the order in which they were first published, the order in which Lewis wrote them, was NOT the order of the set you have. The publishers decided to re-arrange them.
The publishing date means nothing. Lewis admitted he didn't write them in order. Years after he died, the publishers rearranged them in the proper order. There is a reason why there are book numbers on the spine of the book. If you read them in numerical sequence, you can go through Narnian history, much like reading from Genesis (which talks about creation) to Revelations (which reveals things to come in history, including the "Final Battle" aka Armageddon) in the Bible.
Why won't you admit that the books were originally published with The Magician's Nephew as the intended (at the time) Book 6?
Publishers have rearranged them, and until some time in the 1980's, people did NOT read them in chronological order, and had NO problem understanding them.
Did you have trouble understanding Star Wars because Revenge of the Sith wasn't made until 25 years later?