Why would you expect the actual fuel burn to differ from that indicated, unless you know that a particular a/c has unreliable fuel flow gauges?
That's one of the advantages of having a fuel flow indication: you can, in a/c such as the 76, juggle fuel burn vs groundspeed on long legs where you may be on the limits of range. Of course, with a 76 you have the Vne factor to contend with, along with headwind and FF. With the EC-155, you only have headwind and FF; TAS will only get better with altitude, up to about 10,000. Sorry, was that a plug?