“They were not written in order!.........”
And... The original was published as serial short stories in Astounding Science Fiction magazine.
Asimov, in an interview, once lamented that he would get letters from fans complaining that in one book “you wrote that so-and-so was this-and-that, but in this other book you wrote that so-and-so was this-and-this”
“Give me a break!” he exclaimed, “There’s 50 years between those books and I don’t have the time to re-read them all!”............