It's not only on the B777, 75 has this as well. Don't know about other types. Red somewhere in the book or CBT that it is an autothrottle software thing : when the speed is below the bug the throttle corrects faster then with the same speed deviation above the bug.
It helps also if you select descent, descent now +exe in the CDU just a little bit before the VNAV TOD. Instead of going a bit over this TOD and then "diving" to pick up the VNAVPTH and thus increasing speed the transition is smoother and the speed closer to the bug.