I think that the computer's fuel consumption statistics don't actually come from the tank float. The consumption estimate comes from the injector duty cycle and fuel pressure.
The computer probably has a map or graph between duty cycle and fuel pressure which tells how much fuel the injector will spray at any given cycle and pressure. Using this it continually estimates how much fuel it has sprayed since the mpg calculator was last reset. It then takes this amount and divides it into the miles traveled since reset to give you the miles per gallon.
There are definitely going to be inaccuracies in the graph between the expected fuel spray and the actual spray that occurred so the MPG estimate will never be 100% accurate but I wouldn't expect it to be anymore than 5-8% off.
Also as previously mentioned when doing your own calculations you shouldn't use your entire tank capacity as the fuel consumption. It would be better to use the amount that you actually refilled. As long as you refill to the same point in the tank every time this should give you an accurate reading of mpg. Understand that you might not get the exact same fuel level each time though which means there will be slight error in this calculation as well but for all intents and purposes is accurate enough.