I believe it is not so much the curvature of the plane as it is the curvature of the overhead compartment door. The compartments seem to be shallower at the top and deeper at the bottom. If you have a bag that is very long, it might not fit wheels-in due to the height of the bag blocking the door from closing. But if you put it wheels-out, the wheels fit in the deeper, lower part of of the compartment and you gain a couple inches of clearance up high where the compartment is shallower, allowing the door to close.