Scan Gauge Fuel Offset Questions

[GasSavers_goldie]

I've been running a SG for awhile now with good results. However, I seem to need the collective assistance of the memebers to assist me with sorting out a perplexing dilemma and increase the accuracy of my readings. When I fill my tank I adjust the SG to match the gallons pumped and the fuel offset percentage changes, sometimes dramatically. This in turn effects the MPG readings of the SG. I've listed the figures for the last few fill ups in the hope it will assist you. And, as always, thank you in advance for your assistance.
goldie

Scan Gauge Fuel Tank
Tank - 599 miles 606.3 miles (odometer)
22.9 gal. 22.3 gal.
5.3% offset

Tank -538 miles 549.6 miles
20.7 gal. 21.6 gal.
8.8% offset

Tank -500.3 miles 533.6 miles
20.8 gal. 19.7 gal.
3.9% offset

Tank - 562 miles 569.1 miles
forgot to note gallons 22.3 gal.
11.1% offset