There's penetration and there's over-penetration

While penetration plays an important role in bringing down game, it isn't the sole factor to consider. Just as important, if not more so, is the amount of energy transferred from the pellet into the game. Call it shock factor if you like. A .22 that stays in the game will transfer 100% of its retained energy. A .177 that passes through carries away some of that energy. You could argue that a really hot .177 at 1100 fps and one at 900 fps, both over-penetrating the target, a probably doing equal damage. In that case, the extra fps is wasted to a degree.