Major issue I've seen with most 20/21/22" carry-ons is that the manufacturers usually take their measurements from the INSIDE of the bag and specifically exclude any exterior frames, handles, accessories, etc...As such, my 21" bag has manufacturer provided dimensions of 43.5 linear inches (within regulation) but the ACTUAL exterior measurements, including the external frame, is closer to 48" (NOT in regulation).
Yes, this is a continual problem with wheeled carry ons: most manufacturer's measurement specifications even exclude
the wheels and handle. So the manufacturer's specified measurements seem to accord with the maximum specified for carry on by the airlines,
but once you add the handle and wheels, they are over those measurements. And then to make things more confusing, many airlines will allow you to carry on those slightly larger than specified bags anyway, but you can't always count on it -- you never know when you might run into strict enforcement of the measurements and have to check the bag, which is a real problem if you have things in it you're terrified of losing. So unless the bag is within the airline's
measurements including wheels and handles -- which means a
really small bag (and in fact it's probably hard to find a wheeled bag so small) -- I never feel sure whether I'm going to be allowed to carry it on.
(In my own experience most airlines in the US will let you take these slightly larger wheeled carry ons aboard, but in other countries you are more likely to run into stricter enforcement.)
What's needed is some kind of certification system saying that, "If it has this seal, then it can go on as carry on, period, otherwise not." But I guess that would be too sensible.