Every time King Tubby explains human behaviour, the humans illustrating it all look like the Stinkletons. This is likely because to those cats, all humans look more or less the same. And since the Stinkletons are the only humans with whom they interact on a regular basis, then when they picture it through K.T.'s explanations, all of humanity has to look like the neighbours.
Doubles as a Crowning Moment Of Heartwarming, because for all the times the cats say that Kitty is not a real cat, she never appears in those illustrations, which means the cats still see her more as one of them than as a regular human.
A child that is all by herself, seemingly without any family at all, and refuses to speak is never a good sign. The question of where Kitty's parents are has many rather dark answers, and they get worse when you consider that they probably go hand in hand with the reason why she continuously acts like a cat and can only make cat sounds.
Considering some cats are already pretty old, chances are Kitty will outlive them. Timmy Tom will technically become an adult cat (or at the very least, the cat equivalent of a teenager) in just a few years whilst Kitty remains a child. Lord help King Tubby who may have to educate her one day about the differences between human and cat life spans.
However, the Nazz is implied to have lived longer than average when he mentions it took him 25 years to learn piano, so cats in this universe may have enhanced life spans compared to real life.
