I don't know about the FMC on the B757/767 but on the MD-11 there is a difference between setting the speed on the FMC and setting it on the speed IAS window:
If you set the speed on the FMC and therefore fly this 'managed' speed, the autothrottle system works in a kind of 'smooth mode', trying to fly the FMC selected speed with as few thrust changes as possible, instead the aircraft is 'oscillating' around the selcted altitude (just a few feet in each direction) to maintain the FMC speed. Thrust changes are only applied by the system when really needed and then just very smoothly.
On the other hand, if you select the speed on the IAS window, the 'smooth mode' of the autothrottle system is disengaged, and the autothottle system is constantly adjusting thrust to maintain the selected speed.
Maybe the ATS on the B757/767 works in a similar way? Just an idea...
Regards,
DBate