I have not seen that behavior. It also does not make sense to me. Maybe I will see it some day and can figure it out.
Armies do not move at different speeds. All armies move 5 hexes per turn. However, which army moves next is randomly chosen with all armies with a move left being included in the choice. This gives an illusion of moving at a different speed, but they all move the same number of spaces per turn.
The levy of each castle determines how many men spawn at each castle. IF you allow the game to balance the map before starting or play a random map, every player castle can have a levy between 9 and 12. Rebels are something like 3-9 off the top of my head. This levy and the start garrison can also be set in the map editor, but I strongly recommend letting the game balance it.
Thanks for playing and for the feedback!