Marketing. Actually, from the 7.3 FX up to the 7.7 FX (the 7.9 is now called a road bike as it's a carbon-frame) the frames are virtually identical. I have a (former) 7.5 FX that is better decked-out component wise than a 7.7 FX. Maybe the 7.9 FX as well - I haven't checked the components lately.

So if you're thinking of rebuilding one with new components, I suggest a 7.3 FX as your starting point. But do as pleases you. These bikes are fun above all else.