Its normal.
the mac can consume something like 90 watts, the charger can supply only 85 watts. I.e., less than the machine can use at full noise...
If you run it at 100% load with 3d video etc then the battery will drain a bit while on AC.
You'd have to play 24hrs straight or more to drain it fully though I'm sure.
My 2011 does the same, also in handbrake.
I wish apple made a bigger charger, so the battery could charge faster while in use, too. If you were to say, plug into ac at 50% and fire up a game it will take forever to charge.