They are more sensitive when for example the inner mouth is modeled closer to the outer skin.
Removing SSS from the inner mouth solves the problem.

That is why some figures get them, and others do not.

Poser 1, 2, 3, 4, 5, 7, P8 and PPro2010, P9 and PP2012, P10 and PP2014 Game Dev
"Do not drive faster then your angel can fly"!