Is not fuel burn a derivative of fuel flow, eg...400kg/h? In all my years I have never heard of fuel burn being "measured near the nozzles in the engine". Could you please elaborate.
I'm bettin' 172 is referring to the HMU output transmitter, located a little over a foot upstream of the nozzles.
Regarding 172's comment on draining/returned fuel causing a higher reading, I am not so sure about that one either Cap. I've always understood the change in levels was due to the FQIS averaging out the probe readings more accurately with the aircraft still and level on the ground - with the main tanks having more variation than the center.