My 2 cents:
Chances are around 90-95%, if you look at your thumb from the fingertip towards your hand that your thumb is oval shaped. If not, when you use any pressure to hold onto the ball, it will then be oval shaped. If you have a round hole, without tape, front or back, then, when holding the ball, you have a space there, requiring more than the minimum amount of pressure to hold it, whether you realize it or ot.
If you're in the small percentage that does have a truly round thumb under any circumstances, you are indeed rare. Since you say you're using both round and oval thumbs, chances are pretty good you have an oval thumb and should be using such, for a proper fit and grip.