The likely explanation is that your supporting fire is going into your unit's back while the enemy troops have a shield to their front.

Proper support with foot archers is a surprisingly neglected topic in the forum. About the only guidence I can give is that I usually put my foot archers in a long, thin line if they are behind my melee infantry and get them closer. This seems to relieve much of the "undershoots" that cause friendly fire losses.