I had the same issue with my 2011 X5 50i, and now with my 2012 50i. Always seemed odd to me that the tank would be almost empty, but it would never seem to use as much fuel to fill as the stated fuel tank capacity suggested it should.

I did try topping up the tank after the nozzle clicked off a few times, and was able to squeeze about 5 litres (just over a gallon) which made the fuel guage show just over the full mark. Seems it just clicks off too early for whatever reason.