isn't it the case that they (the real carry on sized luggages) don't just say 'carry-on' luggage in the ad but 'IATA cabin size' as well?
I started to worry since I ordered a Rimowa Classic Flight but I checked and its exactly 55, 40 20? well to be honest it says 56, 40,5 20,x but… I guess it won't be by coincidence half centimes too big or so :-). It's only ryanair anyway that really acts stupid about it.