Perhaps the biceps isn't as insufficient during pronated grip, seeing that supinated grip really puts the biceps into severe mechanical disadvantage by having the arms too elevated at the beginningof the motion and having the elbow already flexed just when the arm is coming down and the biceps would normally be ready to kick in. Seems like the biceps are abit more stretched and able to work when grip is pronated.
One of the functions of the biceps is the supination of the forearm
, and flexing the elbow with a supinated grip improves the biceps mechanical advantage. The standard biceps curl can be done with more weight than a reverse curl.