HP Pump speed
Phil
If it's anything like the Fuel Control Unit on my aircraft (which also has R-R engines), the HP Pump pushes out considerably more fuel than the engine needs. A variety of spill valves bleed off fuel for various reasons. The most important is a system that ensures a constant pressure drop across the throttle valve. Adjusting that valve then produces a given flow for a given setting regardless of what the pump is up to (within limits obviously). The electronic side of the engine control adjusts the throttle valve, and a monitoring system tells the electronics whether or not the desired result has been achieved.
What seems to have happened here is that the desired result was not achieved, so the electronics continued opening the throttle (that's all they can do in the last analysis) in order to get the desired result - which didn't happen.