My understanding is that when the belt is under tensioned it loads the nubs differently & they become like gear teeth & inevitably rip off as it is only the nub tip that is being loaded. Proper tension loads the entire belt structure from the base of the "v" between the nubs & the nubs themselves become something of an anti slip feature.

I've also been told that when properly tensioned, most bikes/belts are about in tune with the "D" string of a guitar.

The rub is when selling bikes most consumers notice the slight running resistance of a properly tensioned belt drive & being Leary of "not chain" already, they will not purchase the bike unless the belt is loose like a chain. (About 1/4 inch play up/down.) So bike shop mechanics set them under tensioned on purpose because that is what the consumer expects. The consumer who does not know any better, suffers short product life & less than satisfactory performance.

I may be wrong & I welcome any correction.