I usually round mine up to the next buck or two, if it lets me. That can cause discrepancies on individual fillups, but not your overall average. The meter in the car usually goes by what it thinks it should be.
t's difficult to get an exact reading every time. Are you filling up to the top every time? If not, your fuelly.com calculation may be off. Since it uses the number of gallons you put in your car, you need to know that you're being consistent on your fillups.