RE: US carriers often ignore their own limits and let people carry way too much onto the plane. If you are boarding late they then want you to gate check your bag - even though it's within limits.
If EVERY passenger boarded with legal maximum carry-on baggage, narrow body equipment (e.g. A320 or 737) with 3+3 economy seating will NOT be able to accommodate everyone's carry-on in the overhead. If the economy section seat pitch is 31 or 32 inches, and everyone's carry-on is 14-inches wide, with three abreast seating, you cannot get three 14-inch wide bags into 32 inches of overhead.
On then there is the passenger with the stuffed roller bag that has to put their bag in length-wise taking up 22-inches of overhead.
And of course, econo carriers like Vueling have even tigher seat pitches at 30". Vueling A320: http://www.seatguru.com/airlines/Vueling_Airlines/Vueling_Airbus_A320.php
PS RE: The company website says it only allows a bag with dimensions 55X50X45 cm and weight not more than 10 kgs .
Both Seatgaru and Vueling websites say that the maximum size of carry-on is no larger than 55x40x20 cm. Maximum with of 40 cm s less than 16-inches. That would mean in a full 3+3 configuration, the overheads would only fit 2 of 3 passenger's carry-on.