I’ve noticed this as well. The first time I saw ‘Predators’ I knew there was something that didn’t sit quite right with the PIS design.
The second time I watched the film I figured out what it was. The P1S seemed to be unable to shut his jaw completely, but oddly he seemed to still be able to work his mandibles. I thought along the lines of Berserk_ wolf, in that perhaps this (slack jaw) was due to the stress and hardship the poor guy had suffered…but that seems less likely to be the issue behind it than one of design.
Also, as for why others aren’t able to ‘get down’ the proper jaw/mandible articulation on theses creatures - do you think it could be on purpose? Maybe the particular mechanics we see in the original Predator design are copyrighted or something? And maybe that’s why no one else seems to be able to recreate it.
~Estelle