I had just formulated my reply when I noticed johntullamarine's excellent opus had beaten me to it. I'll post it anyway:
Using the conventional mathematical calculation method for working out C of G one can finish up with very large-number moment values. These numbers are often divided by 1,000 to make them more manageable (as at the bottom of your graph).
The author of the spreadsheet has chosen to follow this convention and has created his 'constants' by simply dividing the arm by 1,000 rather than the resultant moment (the answer, of course, is the same).
The fuel 'constant' is the fuel arm/1,000 factored by 6 because you are entering a value in gallons, not pounds. The spreadsheet does not, as you might expect, turn your gallons into pounds and then multiply by the arm, instead it's simply frigged the arm so that gallon values can be computed directly.
MT