A reason? I can name several off the top of my head:

- They were once normal Narven, but they fell under the influence of dragons and adopted some of their powers, acquiring only a small taste of it. Even so, this taste was enough to change their physical form.

- They split off from the rest of the Narven at an earlier point in time and evolved that way over millions of years in the heat of the desert, very much unlike their swamp-dwelling brothers.

- They were magically altered by some mages or a genie or some mysterious force in the desert. 

- If you don't like any of the above, suggest a reason. You already have most of them in a swamp, near a desert, so why not have a few of them wander off into the desert and be changed by it? That actually seems like a very strong reason, come to think of it. You could easily make the desert Narven "dracons." Animals living in different environments have adaptations to help them fit into their climate, even if they are part of the same overall species or genus.