Frontier Airlines got in trouble last year because their carry-on size rules were so confusing. I read that they make most of their money from these extra fees, not the ticket price itself. It feels like they set you up to fail with those tiny sizer boxes at the gate. I always measure my bag at home now, but sometimes it still feels like a gamble.
Yeah, their whole system feels designed to catch you out. I measured mine perfectly and still got hit with a fee last trip. Just another way to squeeze more money from you.