Other mistake: When Edgar goes to retrieve his hat and umbrella, he uses a fishing pole to snag the hat from Napoleon's head. You see him reeling in the line when it should be reeling out (counter-clockwise vs. clockwise)-the next shot shows the line going down. This happens twice.
Other mistake: In the scene where the three kittens are playing train on the train bridge, Barliose is standing in front, then Toulouse then Marie. After Barliose says, 'Marie's the caboose' you look at Marie's reaction and Barlioses is painted in front on her instead of Toulouse.
Chosen answer: It is the style of the artwork. Similarly, it is not a mistake that each individual hair is not drawn, the eyes are not really proportional to the head, etc.
Myridon