Hand calculations don't lie as long as your odometer is accurate. I'm not 100% sure but I think they are also subject to about 95% accuracy factor. I believe that fanactics that are into road rallying have a seperate odometer that is calibrated to a much higher degree of accuracy.

It could be that people that are not getting what their electronic display is reading (either high or low) are off because their odometer isn't reading correctly - or it could be the computer that's off - or they're both off.