Behind the scenes we store everything as US units so we can easily switch the units for display across the site for all vehicles. In the process of that conversion you do lose some precision if you report in kilmeters or litres.
This structure is a core part of our system and it's not something we can change. If you need absolute precision Fuelly probably won't work well for you. It will give you a good sense of your fuel economy within .01 either way, but I can understand your frustration if you need something more precise.