On my few flights, I've been noticing while in VNAV climb that the aircraft will continue to climb at the VNAV speed (knots) and not switch over to mach number until its a couple of thousand feet above crossover altitude.

It seems to finally switch over at about 31000 feet altitude, and since it followed the speed in knots until this time, the actual mach number is slightly higher than the VNAV mach number value and the aircraft will climb fairly rapidly for a thousand feet or so to meet the target mach number.

Has anybody else noticed this behavior?