I asked this question to myself everytime and I never found a good reason. When I was SunWing this summer to CUBA, I need to pack my luggage with so much room inside where stuff just move around.... and I still end up with 53.5 lb at the airport. Maybe it is the way I always pack during business trip, small and compact, so I end up be able to pack more... and my luggage is only a 28" or 29" one.