What is the difference between a TREK 7.2 FX and a TREK 7.3 FX? I don't understand the numbering system on TREK bikes. 

Is it the lower the number the better, or the higher?

What is exactly is the difference?
This higher numbers are upgrades.  Trek website

Upgrades on 7.3 FX from 7.2 FX

IsoZone grip and handlebar system
Bontrager Race All-Weather Hard-Case tires
Alloy FX Fork