I am sure everyone has had this frustrating experience. A bag labled as carry on with specific dimensions and it is over sized when you actually get it. Here is a great link from consumer reports that talks about how manufactures measure their bag and how the airports measure it.
You would think they would be the same but they are not.
Recommendation measure twice buy once!
http://www.consumerreports.org/luggage/When-carry-on-luggage-isnt-carry-on-size/