Pandora!

Well done! You've noticed something interesting and useful. I haven't really delved into Pandora's Boxes much, so I didn't pay much attention to it.
As I understand it, if you take a Pandora's Box near an enemy castle, the units inside will be similar to the enemy's. That is, they won't be tied to the hero's/player's race who took the Pandora's Box. They don't seem to be tied to the terrain type because one terrain type can be related to several races at once.