BMWs calculate the mpg over the last 40 miles so periods of sitting doing nothing will affect it far more than the systems that average all the way back to the last reset.
Similarly, in Andy's case, it's only got, say, 6 miles in the average bank before it sits doing nothing which will soon clobber the average. And garages generally leave cars idling throughout the MOT to get them nice and warm for the emissions test so it's a lot of idling.
Besides all that, petrol engines are hopeless at idle. Diesels are completely the other way and will sip very parsimoniously when doing nothing. Friend used to work on the traffic lights and, on cold days or when the van battery was getting past it, he'd leave it idling for hours and the fuel gauge would have move imperceptibly.
