scifi
The engine speeds discussed and displayed around page 51 of the report were derived from audio recordings. The graphics show only a small portion of the vertical axis.
Elsewhere in the report it is claimed that a pilot would not be able to sense or hear any variation in the thrust level.
The data was arrived at through minute analysis of the audio recordings in an effort to explain why the aircraft did not achieve the target (gate) apex height and speed.
Historically there have been unexplained losses of thrust from similar engines while in military service but Rolls Royce, the manufacturer, have never been able to establish a definitive cause.
The engine was not spooling down towards zero, just a fall off of a few hundred rpm or so. This is enough to indicate that the thrust was not at maximum throughout the climb, or at any point in the climb. There appears to have been a reduction in thrust at two points in the climb. Rolls Royce have been unable to find any fault with the engine that may have resulted in less than full thrust being developed, nor for an intermittant reduction in thrust.
The alternative explanation offered is that the pilot did not initially select full thrust when commencing the climb and adjusted the throttle during the climb. There is no visual evidence from on board cameras to support this view and it would go against normal practice when performing this manoeuvre.