A THREAD FOR NEWCOMERS!

Question: What determines the initial troop placement before a battle? In Heroes 1, 2, and 4, if a unit is placed in a specific slot, it will always meet the enemy in the same location. In Heroes 3, however, the last slot on the bottom row of the battlefield doesn't stay put; it somehow moves closer to the center. So, I'm asking, how does the unit's horizontal position in the hero's garrison affect the location where the unit will meet the enemy in battle?