However, I always get a bit frustrated when trying to answer it, because I know that it is really not the right question to be asking.
When I ask these questions it is to interview you (or, really, the company). What is the company implicitly and explicitly teaching it’s employees about how decisions get made?
In other words, I already know the answer, I’m seeing if you do!
That being said, I would guess that the more inexperienced you are, the more earnest the question asker is about the question.
You’re precisely right—I’ve asked it myself in order to see if the company operates in a reasonable manner or not :)
Your second point is spot on as well—it mostly comes from more junior candidates (which, numerically, is the bulk of the people we hire), and from the follow-ups I get, it does often seem to be asked in earnest from that population.