Who really knows, I have a Briggs International carryon that I love and recently on an AS 737-400 couldn't fit it in the overhead wheels first, it had to go sideways. How an international sized carryon which has a smaller standard than the US doesn't fit in a US domestic jet is beyond me. Created quite a stir in the F cabin from SFO when 3 passengers had carryons that didn't fit at all. Those planes suck bad.

Side note, do people really like the 737? The fact that AS brags about being all Boeing 737 seems a little silly to me.