no its lvl progression. You have your starting area and everything in that starting area will be around the same level as you once your ready to go to a new area rinse and repeat. It will be more detailed then this but i did not want one big paragraph of explanation
As Osteo said... Its the MMO approach, you might have seen a "brown bear" as a lvl one mob... but in the next zone / area a "Yogi Bear" may come out with his basket and whoop your butt if your not up to the right lvl.