The DTE readout is based upon the fuel gauge sender signal and is subject to the inherent innacuracy of these devices (which, from previous threads, suggests that they are generally calibrated to read low so that you don't run out of fuel). The readout for DTE will drop quickly when you are idling as it seems to calculate this figure fron the indicated fuel tank contents and the instant litres/100km figure which, when idling, is effectively infinity! The only figure which seems to be 'accurate' is the readout for litres used as this seems to come, not from the fuel gauge sender, but from a summation of the actual fuel which has been metred through the injectors and stored in the computers memory. So long as you reset the trip when you filled up and know how much you had in the tank to start with (i.e. 75 litres if full) then you can accurately calculate what is left. I think one of the reasons the gauge is pessimistic is that it is bad news (for all sorts of reasons) if you run completely out.
Oh yes, and I also regularly get 850 - 900 kms out of 75 litres but if I go near town this comes crashing down!