It could be related to the onboard computer software. You may be averaging the lower mpg number for the entire tank, but as with all statistical analysis, the larger the sample the more accurate the results. So, when you have gone through a full tank the computer has a larger sample to use to calculate mpg. One way to test that is to drive until you have used about 1/2 tank and then do a manual calculation and see if it is lower that the FE meter's reading.

Good luck,

Mark Owens