If you read the posted article, you will see why the initial leak could have been very small, but increased greatly day by day. The article also provides factual evidence that this is what happened.
So it could well be that the first day there was only a little leak, but within days the leak became much bigger, as the high-pressure oil wore a bigger hole in the cracked riser pipe. (The article mentions a pressure at the BOP dropping in half over that time period, indicating that a lot more oil was escaping later than earlier).
Your complaint is like saying someone lied to you because they told you at 1pm that there were 2 inches of snow on the ground, but when you looked at 8pm there was 10 inches.
Not necessary for the oil to "wear a bigger hole in the cracked riser pipe" to account for the increased flow. More likely is erosion within the structure of the reservoir, with the formation and widening of channels in the rock structure (which, as I understand it, is a sandstone material, and easily subject to abrasive erosion).
Eventually, the flow from the formation would reach the point at which the back pressure from the BOP/riser/etc. became the flow-limiting factor rather than the cracks and crevices in the rock. At that point, the flow should stabilize.
The guys over at "The Oil Drum" are saying that this is more likely than riser erosion.