I believe it's because the fuel tank is such an irregular shape that the fuel range is only accurate when the tank level has dropped to around half empty. Above half the range is given as an estimate, indicated on the display by the symbol >, below half the reading is accurate. I have a 10 plate F800GS and it does have the fuel range facility and I have found it to be really useful on a long run despite the above.
I think BMW had a large number of complaints from owners believing that the display was faulty when the range on a full tank appeared to read constantly around 125mls and then would start to reduce as the tank level went down so they simply did away with the facility.
I rode a 12 plate a couple of weeks ago which did not have the range facility.