Ok I have a theory, and hear me out on this.
It's all about the desk and chair. If your desk is low and chair is somewhat high, your thumb should rest on the curved part of the space bar. On the other hand, if you have a higher desk, your thumb would hit the edge of the space bar, making flipping nicer. Same applies for keyboard angle and wrist rests. High deskers like higher keyboard angles, and low deskers like lower keyboard angles and wrist rests
That said, #flippedbarmasterrace