Interesting - I think it needs to be further tested, since there could be some
further explanations for the results.
The next test should be constructed so that every other cup is sprayed, and the opposite one buttered, to eliminate the possibility that the center position is what caused the differing results.
I think the reason for the results are not that the sprayed sides rose more evenly, but that the ungreased ones had something to cling to and could climb higher and quicker, making an overall more fluffy and tall muffin.
I think further trial runs are in order........
