On a recent trip using Lufthansa, my 21-inch roller carry-on was accepted without question for 3 of 4 flight segments. Leaving Barcelona, Spain, however, I was stopped and instructed to try my carry-on in the size frame. My bag did not fit that frame. OK, "boarding pass and passport!" I complied and the offending carry-on became a checked bag. In the meantime, many other "carry-ons" MUCH LARGER than mine were allowed in the airplane cabin! Why this discrepany/discrimination between types of luggage?? If one type is "too large" for carry-on status, then why not all types, i.e. specifically duffles? There is NO way many duffles would fit in the size frames the airlines use for carry-on sizing. On a trip with Lufthansa last year my same carry-on was not questioned anywhere. What's Barcelona trying to prove with their obvious discrimination?