Are you looking at the #1 or #2 average?
#1 average is just the current trip or the last X miles (50 sounds about right), but it will only take whichever is the shortest.
I'm pretty sure that the #2 average is based on the average since #2 trip computer was last reset, I tend to reset mine every tank.
Speaking of which, recent average seems to be down to 33.x mpg, I had been stuck in some pretty ugly traffic jams, but have subsequently had a good long motorway blast and average only crept up to about 35mpg, which is a little strange since that was mostly 100% legal-speed and certainly no silly business... so would have expected high 30's really.... have Audi sabotaged my car?!?