How does the TL calculate its "Avg MPG" on its Trip Computer (2007 non-navi)?

I know the number reported is inaccurate because I always get a different number (usually lower) when I use a calculator and manually divide the number of miles driven on a full tank and divide by the number of gallons needed to fill up.

For example, after filling up my tank last week, I immediately reset the Trip Computer. While filling up yesterday, the Trip Computer said I averaged 22 MPG on mostly local driving. I drove 280 miles and filled up 13.8 gallons, which is really 20.2 MPG.

Can anyone figure out exactly why there's a discrepancy between the reported 22 MPG and the actual 20.2 MPG? I don't think it is because the Trip Computer is malfunctioning; I think it has more to do with the way it was designed to make the calculation.

From what I've observed, the Trip Computer calculation refreshes every 20 seconds. If I reset the Trip Computer immediately after I fill up at the gas station, I'm always stuck with a few 5-8 MPG readings as I crawl out of the gas station and wait at the signal for a few minutes. These initial numbers, of course, completel kill my average MPG number.

I can't put my finger on it, but the way the computer averages the numbers looks fishy to me. Thoughts?

 

what throws it off is when you are coasting, the PC thinks you are getting 99 miles to the gallon when in reality you are still idling getting around 30.