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?

Your title should read "crossover speed".
Yes, I have also noticed this.
FMC set to climb speeds of 310/M0.786
However, the plane continues to climb at 310 until reaching a speed of just over M0.80. Only then does it pitch up (in VNAV) and slow down to M0.786 whereupon it continues to climb at the FMC mach speed.

CS, could you plz investigate this issue?
Thanks.

Anyone else notice this in FSX?