Python intersection and difference sum doesn't give me the actual number of the original set

Question

I have two lists, one with old IDs and one with new IDs.

I want to get items in common and items not common.

The new_Items list has all new ones. The old_It

Answer 1

The list had some repeated items, that was the problem.

So the set cuts these repeated items and that is why printing less numbers than I expect.