What Slomo says is correct for belts like fan belts that always drive and do not clutch.
Mower transmission belts have deeper pulleys because the belt has to go slack to beak the drive and the pulley has to be deep enough to prevent the belt jumping off the pulley.
ZTR trans belts are the exception because they are always driving.
All V belts wear on the V sides because they are the contact faces.
Usually the wear will allow the belt to sit deeper in the groove so it goes slack then it slips so it wears even faster.
Very rare for the belt to actually get that thin that it runs on the root of a pulley.
The exception to this is the varidrive pulleys and the stack pulley on the engine.