My X5 has been doing that for over 9 month.. finally got fed up with it.

My was 7/8th full.. and at times.. its 3/4 on the fuel guage... even when fill it up completely.

The dealer took 2 days to fix it. They made an case with BMWNA to ask for help.

Replace both Fuel sensor was the final solution.

They found out that.. 2 fuel sensor had 2 different voltage reading.
and when they hook it up computer, 2 different sensors were read diffrently on the amount of gas left.

Once those 2 sensor is replace, everything is back to normal.