If it was a clean barrel when you started, chances are good your 1st assumtion is right.
I had a pol months ago now about cleaning, when's and how often, and the general consencus is they like dirty to shoot good.
And it holds true with my barrels, I was surprised to find I actually over fouled my 7-08 testing some barnes TSX, so much so that it schewed my test results, but that's another thread, Yup, they like some fouling.