I think it's something we all felt the effect of, but couldn't place. I didn't consciously realize what it was until Prime 1 recreated that expression.
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Normally I'm not a fan of companies using other collectibles as reference (and Prime 1's Jungle Predator
is a bit too stocky from some angles), but the benchmark for portrait accuracy was definitely raised here. They even picked up on the mandible fangs overlapping. The top left (our left) tucked behind the bottom-left. The bottom-right pointing inward. When this bust first released, it was thought to be a QC error!