Merc's-Single Player

I'm up to LvL40 in single player. Woot. Anyways, I've had a merc w/ me the whole way and it's basically like having another player, we own everything. Well he was a concentration merc, which was nice cause I had atleast 75% chance to hit, pretty much everything. The minute that I entered the Worldstone Chamber to woop Baal, he morphed into a conc/might merc... Is this normal? I've had merc's online, but I always lvl them up from nightmare, never all the way.
Also it seems that his Might adds to my damage, along w/ my fantacism. Does it damage stack? Say his might does 50% ED, and my fantacism does 75%, will I be getting 125% or just 50%, and then 25% more after that? Does that make sense?

Never heard of them changing, you just probably bought a new one from nightmare mode and didn't know you did. And yes, might stacks with fantacism, which makes it very usefull.

No, he changes randomly! Like he'll be using Concentration one minute, and then he'll switch to Might. Its crazy. It started when I got to Baal in Norm.