In the early 1970s as the first fuel crisis started, one of our aircraft arrived at base from Central Africa with more than double the expected fuel remaining. Operated by a Training Captain, I was asked to find his technique, or his short cut, so that it could be applied by all crews. We had only one aircraft which was calibrated in lbs, the rest of the fleet were in kgs. ( Just guess... The best Training Captains really ARE human, too!)
Another fleet night-stopped in Africa, so Captains used the time to have the exterior of the aircraft cleaned and they paid for this out of the Captains' Flight Funds. On a range- critical sector, this reduced the number of Tech-stops. That is until there was an internal accountancy debate, "Should this be costed to Engineering or Flight Operations ? "
At the time we were not required to tell the Company why we had overshot or diverted... Just not to do either, too often. I would guess that these occurred on far less than 1% of flights, and probably for what seemed, at the time, to be good reasons. (Hindsight might have differed.)