Thanks Lisa46, I don't think that's whats happened. Both cats have 8 traits. Since my post, there are now two kittens and my sim knows their traits and they have both developed a new trait which she also knows. She spends a lot of time with them. She has the cat lover trait. Any other ideas?
I've had this happen with families as well. Sometimes one of them doesn't know one of the other family members traits. Unfortunately, the 'brain chair' (from University) won't allow use on family members, children or pets. So Sad.