I am hesitant to handle this--my understanding isn't as precise as I'd like.
Do you do the concentration curls with the arm completely vertical inside the knee, or at a bit of an angle? If you do it at an angle, I think the answer lies in an understanding of the concepts on these 2 pages:http://exrx.net/ExInfo/Muscle.html#anchor1279543http://exrx.net/Kinesiology/AnglePull.html
I think that in the concentration curl, the most efficient part of the muscles' range comes into play when the gravity force vector is the greatest, but with the simple DB curl, the muscle is contracted beyond it's most efficient point by the time the gravity vector is maxed.
Most lifters would not notice this, since they swing the DB curl, developing momentum with the swing at the bottom of the movement. The fact that you can't move more weight with the DB curl than you can with the conc curl shows (I think) that you are using strict form on the DB curl.