[Avshalom]

I think in this case it's mostly down to the fact that the robot (fore)head stays perfectly still while the human head wobbles about naturally.

[luchak]

It looks to me like the robot just exhibits less motion in general. This might be because (as far as I can tell) the authors optimize only to match a set of static poses, not for physical or perceptual realism of motion.

[finnw]

For me it's most noticeable in the lips - they just don't move as much as they should.

[bunderbunder]

Especially at spots when there's a labial sound in the speech. Another phrase that comes to mind is, "gottle o' geer".