[chatgptexpert]

There is also prime rib, but I don't know of a rib that is only divisible by itself and 1.

The problem is that human language is very imprecise and requires too much context to determine the correct meanings of words. If the whole corpus of training data did not inherently have this ambiguity, training an AI would be much simpler and I think such simple ambiguities would be eliminated. The author did not specify which definition of "prime" in the prompt. By clearing stating "prime number" there is no ambiguity, and ChatGPT gives the correct answer.