I'll take a guess.....something to do with keeping the performance data valid.
ie the data assumes a certain thrust, it obviously can't come on instantaneously, but at the same time if you brought it up so slow that you only reached planned thrust at Vr then you would use more runway. My guess is that they calculate the a/c accelerating according to reaching planned thrust at 60kts.
As an aside, sometimes when there is a heap of wind on the nose the auto throttle goes into 'thrust hold' before it gets to the desired thrust setting so you have to keep pushing it up manually. Happpy to be corrected by the 'performance gods'
