In multiple cases it was the newly advanced one that was different in kind than others. Toasters work fine under the three laws, even in Terminator the humans are shown with obedient guns and didn't insist on fighting bare-handed.

In other cases, the robot was the same model as well behaved ones, and it had an error making it conscious, or something like that.

You're right that the stories can't all be characterized the way I characterized them. There was a lot of variety, he made a career of them and didn't do it by writing the same story again and again.