Right, so unavailable (partial) is more appropriate than the amazing 0.0L/100km... Maybe?
It's extremely difficult to calculate MPG on a partial fill-up without extra sensors and gauges to know the exact amount of fuel used during that trip.
Fuelly relies on users filling their tank to the top to calculate accurate MPG.
Without the necessary information, MPG=0.
I get what you're saying, but "unavailable" implies that it will become available at a later time, which isn't the case at all.
The main site displays it as blank unless you export your data to a CSV file, which is then given a zero in the field.