The archers were designed to purposely pick different targets as long as they are in range. When an archer brigade is moving towards a target it can sometimes be difficult for the AI to determine the best point to stop and fire. Ideally you want their entire formation to be within range of your entire brigade of archers but sometimes this would put your front row dangerously close to the enemy or if the enemy is moving you might want to fire immediately rather than risking they get away before you can get into position.

That said, I will double check as soon as I get a chance to see if there might be some other problem going on. Thanks for the feedback.