Here's an example with the Galaxy SC models.
SC1 is what you posted.
SC2 gets the better 8-speed hub, and shifters built into the brake levers. Components are generally better.
SC3 gets a carbon fiber fork and adjustable handlebar stem.
The Trek website is actually really good about this within a given series of bikes, it will say
on the page for the 7.4 FX that it has a better fork and drive train than the 7.3 (just an example).