I'm curious where you read that. Far be it from me to be a doubting Thomas, but on first glance I don't know if I believe that. There are reports of rounds (specifically cannon, but the same principle applies I think) going through the tail of the craft, through anything behind the pilot, through the armor, the pilot and through the instrument panel. If it tumbled after hitting the skin I think it'd never have made it through the armor.
Perhaps the smaller rifle-caliber rounds tumble, but the heavier 50cal rounds (with their inertia) continue on path?