I've seen this quite a lot, not just boarding AA flights in the US, but shorthaul BA flights across Europe.

I agree with earlier posts about over-sized carry-on being challenged, but sometimes the staff are a little too over-enthusiastic in targeting pax where they perceive a problem that does not exist.

My roller-board, which fits easily in the overhead bin on every aircraft in the AA and BA mainline fleets, is sometimes scrutinized because of the design. The embedded wheels are slightly larger and have a round cover like a "hub-cap" which appears to make the base of the bag bigger, but in truth it does not.

The dialogue sometimes goes,
Them - "That's too big. You'll have to check that."
Me - "No, it fits"
Them - "It's too big, you'll have to check that."
Silence as I walk past them and ignore anything else they have to say.

They are mistaken about their interpretation of the situation though do not realize it. They are a distraction and delay as I know the bag fits. And finally, they have no authority, even when it is an AA staff member. By the time they realize they will have a challenge if they pursue this with me, they turn to the next victim in line. There are no federal regulations about following instructions of ground staff, only flight crew.

If you know it will fit, and are not running on at the last minute where there is genuinely no more overhead space, just ignore these jobsworths. Let them just antagonize their next victim, who is probably not far behind you.