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: When the three kittens are playing train on the train bridge, Berlioz is standing in front, then Toulouse, then Marie. After Berlioz says, "Marie's the caboose," look at Marie's reaction, and Berlioz is painted in front of her instead of Toulouse.
Other mistake: Everyone stops when Roquefort shouts "Quiet!" to crack the lock, including Edgar. However, since the pets can't be understood by humans in real life or in the movie, Edgar should've been the only one to continue moving, and just the cats should've silenced.
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