There's a level 16 minion that does 7, and one that does 8. Some do +1 damage with combat advantage, most don't. The skeleton minion does less at range than in melee, even though the stats would indicate otherwise.
I think it's just balancing what works. Minimum damage by level bumped up or down 1 (or by giving one more damage with advantage) as needed. That's the only thing I can come up with.