Behind the scenes we convert all data to US English units for storage, so we never have the exact data you entered into the system. We do this so we can show all fuel-ups in whichever units a user wants: US units, UK units, or metric.
And we store all data to a precision of three decimal places. So in the process of converting from the metric units you entered to the US units, we lose some precision. Short of rebuilding Fuelly from the ground up to store data in a different way, this level of precision is something we're going to have to live with. It's not precise enough for exact numbers, but the whole process of measuring fuel economy based on a full tank is more of an art than a science. I think this level of accuracy is acceptable for finding realistic averages.