Question about discrepancy of mileage

There is some bias error. I've been tracking data for 21 months and my bias is 1.6 MPG. The MFD shows 1.6 mpg better than the calculated (miles driven / gas used) amount. My lifetime actual average is 52.6 MPG so the MFD data is off by about 3%.

My data is on my web site Paul's Prius

 

if for example you pull up to a stop light and the ICE still runs, IE: still not into S4, that fuel is included in the MPG readout on the MFD but in reality it's 0 mpg but the mfd calculation doesn't seem to accuratly include it as such. Also another reason is the bladder effect, it's so variable as to introduce some inaccuracy as well. Even if you shut off at the first click any bit of difference in the outside temp has a different affect on the bladder. Then there is the difference between gas pumps, unless you consistantly fuel at the same station and the same pump, you could introduce inaccuracy in your calcuations. The best thing to do is put all the gas vs mileage into a spread sheet and look at it at 1000 miles and see what happens to the difference. Then at 2000, 3000 and on and on and notice what the difference is. Then you could always just ignore it all and just drive the car.

 

I just bought an 07 Prius with very low mileage. The on screen fuel consumption program showed it has gotten 44.9 MPG on my first tank of gas, but when I filled up and calculated it myself, it was only 35.5. So I am confused, and a little disappointed. Any thoughts here?