Car computed fuel mileage a bit optimistic

So pretty much every new car has a readout that gives you the mileage.  It seems that all of them seem to be a bit optimistic.  After going through three tanks of gas on my 2012 Lexus CT200h, it appears to be about 5% optimistic.  The first tank could have been a fluke.  The second tank could have been pump variation.  After three tanks though, it certainly appears the hand calculation (mileage divided by gas pumped) shows the computer is optimistic a bit.