It's called balance. We want consistency, but at the same time we don't want to look at the same three drawings for an entire series.
Atla, for example, was for the most part on model, but the characters would make expressions and poses that weren't on the model sheet. The artists had wiggle room to inject their own style without going too far and being sloppy and inconsistent.
Disney was also really good about this; different artists had different styles that could be identified if you payed enough attention, but they didn't go overboard with it and draw a character's head twice as big as it was in the last shot.