I'm getting constant sneak increases while testing this. As far as I know, sneak only increases when you do damage during attacks undetected, right? It would seem that some of these arrows are hitting and registering correctly as sneak attacks, but without the HUD telling me or damage being taken off...
There are passive sneak checks that occur while you're in stealth near enemies and moving near them as well, I think. I'm positive it was this way in Oblivion.
Erase the people telling the OP that he's poor aim and this thread would be about 2 pages. :\