Autothrottle speed range
I can't seem to find a reference for it in my 777 manual, but perhaps it parallels a function on the A330 which is similar.
That is when the speed is controlled by the FMC, the speed range is narrower than what the pilot is allowed to select.
For example, the FMC can only run the speed up to Vmo-10 while the pilot can manually select up to Vmo. Similar idea on the slow side, where the autothrust will limit speed selection to Greendot (flaps up maneuver speed-L/Dmax) but the pilot can select Vls.