Maybe its because all of your sentences are 60 word run-ons and nobody can read wtf you're saying?
I'd bet your frame isn't true, and you're dog legging/scrubbing/scraping down the road just enough to knock a few mpg off. 16 to 18 could be seasonal variation in diesel fuel, your fuel filter status, or hundreds of other factors.
Unless you fill the fuel until you can see it to the exact same spot in the filler neck every single time with no air trapped anywhere, you're going to get some variations. Are you running the same diameter tire? width? tire pressure? tread pattern? alignment?
Take an average over a few months to see if its improving or decreasing. This is almost within the margin of error of calculation.
2002 Arizona Beige F250 Ext Cab Short Bed. 7.3L 6spd 4x4 140k
2" leveling kit, 285/75 Mastercraft AT2's,Napa BFF, DiPricol Pryo/Boost, DP F5 NS/stock/hi/40/80/120/140agg-lope, Toolbox, Audio goodies, IH shiftknob
muffler delete, CCV reroute, AIH/Foil delete,