That's because years ago they, CO, changed the bin doors on the 733, 735, and 752's so you could put in a larger bag. The one bag I have, but don't use anymore, was specificly made for that larger bin (also worked with UA's modified bins). However when the 737-NG's came on line along with the 753's, they had a bin that was the old smaller size. Of course the 2008 delivered 738's and 9's have a larger bin. I thinks it's a case of blame CO for not being on the ball to make sure Boeing put in bins on the new aircraft that matched the bins they spent a lot of money to modify on the fleet that was in service.