Best thing with a marvel of technology like a 7er is always plug it into the diagnostic. Theres probably a function to look at exactly what fuel level is being measured.

You have 2 level senders on your tank, Maybe one of them is not functioning correctly or the link from one side of the tank to the other is not working.

http://bmwfans.info/parts-catalog/E65/Europe/740i-N62N/feb2007/browse/fuel_supply/fuel_pump_and_fuel_level_sensor/