Although it is quite simple to measure the fuel flow on the ground, on a rig, I can see practical problems with precisely measuring it in flight at the various altitudes. The test aircraft would need to contain telemetry and the data would presumably only be valid for that particular aircraft type. It would perhaps need to be repeated for each type of aircraft the engine was installed in.
Or perhaps I am missing something more obvious here. I would be intrigued to know the answer from the manufacturer.