Continuity mistake: When Stuart's original roadster is first seen in the basement, the ignition of the car disappears in some scenes. It later reappears in the roadster chase scene. (00:28:53)
Continuity mistake: While in Central Park, Snowbell the cat loses his collar and falls into a lake. However, before Snowbell returns home he is wearing the collar again.
Continuity mistake: In the first shot before the boat race begins, Geena Davis is wearing a hat. Throughout the rest of the boat race scene, she isn't wearing the hat.
Continuity mistake: In the boat race scene, before the bully crushes the first boat, you can see his 'henchman' next to him, even on a close up. After the boat gets run over, the kid changes into one with a yellow shirt saying, "Hey you can't do that!" After the bully says, "I just did," he nods over the same 'henchman' that was there before, to shove the kid out of the way.
Continuity mistake: When the Littles have their first meal together, when it shows a close up of Stuart looking at the gravy bowl, his milk has completely vanished.
Answer: Since there no scenes of cats trying to communicate with humans, we don't actually know if humans can understand them or not. It may be that cats can communicate with humans, but simply choose not to.