997 Caliper Installed - Even surface & clearance across the wheel barrel
Due to this caliper has an even surface (ie even clearance) it will only "admit" (& pass the rock through), or "reject" the rock. In other words it's very unlikely to let the rock "roll" between the caliper and wheel barrel.
991 caliper has two-way tapered body
Height were measured from surface plate with digitized Coordination Measurement Machine (CMM)
991 Caliper Installed - Uneven clearance (Large on outboard, tight on inboard)
This tapered body will create an uneven clearance, once a rock is trapped (on lower side-wider clearance) it will roll towards inboard (high side-tight clearance) and get pinched between the caliper and wheel barrel causing damage.