First, just for fun I did a little research...
From the manual, BMW quote 3.3 litres/100 km (conversions - 85.6 mpg UK, 71.3 mpg US, 30.3 km/l), measured according to WMTC drive cycles (World Motorcycle Test Cycles).
These will be performed on rollers (chassis dynamometer) in a test lab, with instrumentation set up to measure the fuel consumption, and also exhaust emissions. I do this sort of work on cars (not so much these days though). The dynamometer applies resistance that increases speed to replicate road driving quite accurately. Over the test cycle the driver (rider) follows a scrolling line graph of speed against time, and accelerates or decelerates etc to maintain the vehicle (bike) speed indicator on the line. There are specific speeds and requirements for gear changes to be made so that cycles can be completely repeatable and comparable each time. Simple? Try driving one! So the fuel consumption is calculated for the distance covered over the cycle.
I'd never heard of the WMTC cycles, but found them in a publication and have screenshots of them. Anyway you can see that BMW's fuel consumption figure is based on quite variable riding.
So, back to the original question, what do I see displayed as the average fuel consumption - usually it is 3.4 litres / 100 km compared to the achieved value (based on the top up amount) of 3.05 litres / 100 km (conversions 92.6 mpg UK, 77.1 mpg US, 32.8 km/l). My riding is a good mix of rural 80 to 100 km/h with hills etc and 50 km/h urban speed limit.
I have never seen the calculated value lower than the achieved fuel consumption.
It's quite possible this is intentional to give a safety factor as the average consumption is used to calculate range for the remaining fuel in the tank.
Also remember that if you don't reset the average fuel consumption when you fill up, the value it shows is for the distance covered since the last time it was reset.