The difficulty level varies around the map (approx clockwise rotation getting harder IIRC) - in the early game if you wander into some parts they will have hugely higher health and damage than you and some of the higher end smart weapons, etc. that don't need line of sight.
I was on a Delamane mission returning 7 cars then decided to wonder into an assault area. Found one guy at level 37. I was level 11, I had 3 remaining out of 7 and his shotgun shot me through containers and brick walls while I was hiding restoring health.