I personally think that anderson wanted the second jaw to be able to pierce the mask in AVP for the main reasons that he was partly bias against the preds, & to lose the somewhat secure feeling that the mask would provide. Think about it, in the special features & in the commentaries, he is constantly making references to the alien movies, about how much he loved them, etc, etc. While hardly mentioning the pred movies whatsoever. He probably wanted to make the aliens' inner mouth seem stronger than it actually was.
For reference, in Predator 2, when harrigan is firing the M16 at the predator in the abbatoir scene, he lands several shots into the preds' mask & armour, & it doesn't even cause a scratch, & in Alien resurrection, when the alien fires out it's inner jaw against the glass of the holding cell, it doesn't leave any visible damage.
If u think about this, an M16 would have at least left marks on the bulletproof glass, yet there was not even a scratch from the aliens' inner jaw, & the predators mask was unscathed by the M16 fire. Meaning that an aliens inner jaw would not be able to penetrate the preds mask if u look at the situation realistically.
On top of this, preds' skulls r roughly 2.5 inches of solid bone on the crown. So not only would the alien have to get through the mask, but the thick skull of the predator also. Realistically, I don't think an aliens' inner jaw would be able to penetrate a preds mask if u take all of that into account.