your number 2 answer is the best one ive heard, and sums it up pretty well. Yes, its true - the big triangle doesnt form a perfect triangle, but the other 2 actually are true triangles. The difference in the slope is what actually causes the hole to form.
A lot of people came up with answers that were very close, but didnt really explain themselves very well.
Good job, here is a free cookie to go with your sandwich.