The head models are different.
To illustrate just one example, here's a generated Bridgeport bouncer with most of his sliders not too far from their default positions. I drew the mouth slider to its lower extreme position in order to show what that one does.
https://i.imgur.com/n3Om0y7.jpg
Note how the mouth sagged downward in the full adult sim, and the philtrum (the cleft in the middle area of the upper lip) became very prominent.
That's one of the pitfalls if you create your sims as YA's. In YA's, the mouth height slider does almost nothing, whereas it has pronounced effects on the head models of teens, full adults and elders.
