If you smash two bottles together, do they both break? I would have expected that you'd always end up with one broken bottle, one whole bottle, and (optionally) a fair amount of your own blood.
Except a car can crumple almost any amount from zero to tiny cube, while a bottle can only be perfect or shattered. Plus, bottles have strong and weak points, and it seems less likely that you'd bang two equally strong portions of each bottle together; more likely you'd be hitting a strong corner with a weak wall.
I figure the same effect would apply to two similar glass bottles colliding.