Ok after looking at the spreadsheet, here's what I came up with... The "speed factor" and the "scale factor" have a dependent relationship. I'm not sure which one is derived from the other, but there's an inverse relationship between them, and they both ultimately relate motor RPM (or stripe count) to distance traveled. Multiplying the two values together produces the same value with one notable exception (see table below)
The exception is the G-gauge diesel, and the missing element is what I'll call the "size ratio." In other words, O scale is 1:48 actual size. G scale (at least as produced by MTH) is 1:32 actual size. When you take the product of the speed factor and the scale factor, and then divide by the size ratio, everything lines up to within 1%.
In practice, I would imagine that varying tire thicknesses, possible errors in the stripe count caused by overlapping tape, variations in the diameter of the flywheel affecting count accuracy, sensor distance to the flywheel, etc., may contribute to more than 1% error.