Trust me, you got lucky, they "randomly" selected passengers with those kind of bags for inspection due to measures, 1mm above? charged! Emphasis on randomly, because the pattern was simple: foreigners? charged, locals? go on. We've saw a poor guy squeeze the hell out of his backpack to no avail, it fitted the measuring box, still, charged because "you had to squeeze it"; guess what, plenty of room under the seat, the guy could put two of them. On me? bag had wheels, foreigner, automatically selected for inspection, took the wheels away, still charged because "it had wheels first". I've flew a dozen companies, not even Ryanair, the low of the lowest treats passengers like this.

Or they do something silly where there is an "Online Convenience Fee" when you buy online, but if you try to buy it at the box office, there's a different "Box Office Ticketing Fee." The event promoter should pay the cost of their ticketing service provider(s) out of the money they make, not have the purchaser pay it separately.