1.7miles more? This is ridiculous.
First of all, how did you calculate your MPG? You probably brimmed the tank and worked out how many miles you had done for that given quantity of fuel?
Problem here is you cannot be sure that each "full" tank of fuel was the same. Filling in the morning or at night make a difference to fuel density and therefore how much you can get in, each pump will cut off at different levels, it depends how you hold the nozzle, also how fast the fuel flows into the tank and how the diesel actually goes down the filler will affect how much air is in the fuel so a "full tank" may actually after a few minutes easily take about another 10% of diesel on top of it.