It was a moderately gusty day, by all reports, so as the A/P and A/T were both engaged (AAIB) the 777 would have been tracking the LOC & G/S with the engines spooling up during lulls and spooling down during gusts. Unfortunately it seems the engines "froze" at a point when they were producing less than the datum power needed to complete the approach safely in the configuration they were in (gear down, F25 or F30), leading to an inevitable undershoot.
Another factor is that as the wind backed off and reduced with decreasing height, it would lead to an effective loss of airspeed that would need to be countered by a period of above datum thrust to bring it back to near the approach reference speed. This effect is more noticeable between 1,000'AGL and the ground, e.g. at 600'.