I believe the bins are slightly shorter in height near the front of the plane (due to tapering of the fuselage). I don't understand how loading the bag wheels first helps it fit better, as on many rollaboards the tallest dimension (i.e. the 9 in the 21 x 14 x 9 equation) occurs near the wheels. I think CO has slightly bigger overhead bins, I seem to recall an article about sizer panels at certain security checkpoints being okay for AA but too small for CO.