KuriousJorj-

I'm rather late in this thread so you might already have your answer to the uneven distribution when you place your legions.

The significant factor is how many troops are in each unit. Groups of units of the same type and number, I've found, will always be able to be set at the same depth. Once you add in a unit or two with a different troop total, the unit depths start getting erratic when you spread them as a group.