I specifically try and answer their questions and “hold back” a bit when answering if I can. I also sometimes will drop a nugget of knowing more and study their reaction. Some interviewers react quite poorly if you hint you know considerably more about a subject than they might want you to know.
I have friends with a lot more education than myself (PhD, masters, very senior) and they say they have to do this A LOT.
The goal is to see how the interviewer handles not being the smartest in the room about everything. This is a big deal because it can suss out whether the other person is “on good behaviour for the interview” but otherwise has an enormous ego during regular work hours.
No one wants to work for an ego maniac vetting out all the A+ candidates out of fear of competition.