Since you guys are on the subject about variance lets look at some more possible problems
My wifes car, 2008 CRV EX, was just filled up:
The gallons used was 13.69. So, if we do the math from miles driven 357.5 divide that by 13.6 we get 26.2 mpg, however, if you look at the top left of the display under "AVG." and above "mpg" it says 27.1 which is what the ecu calculated(meaning the average mpg was 27.1). This is only 0.9 mpg difference or a 3.3% difference but I have notice before it has varied less, more or exact.
So, what does this mean???
Well, I am not 100% sure but I can think of 2 things:
1. pump inaccuracy
or
2. the ecu does not take into account how much fuel is used when starting the engine.
I guess it could be a combination of both.
What do you guys think? If you think about it, the scan gauge would have the same problem since it is reading from the ECU as well.