It could be that the battle raiting was hugly in the rebels favour in the first battle (Crete's Archers are some of the best in the game), it's not just on the numbers of troop in the battle. Were the other rebel armies of a low quality?

It could also be affected by how many of the enemy your general actually killed. In the other battles they may have mopped up just a few routers or something. In the Crete battle was your general wiping the floor of unit after unit of the enemy?

Also it could be affected by the retinutes that your general may have collected after the game.
These can give you general some handy boosts.

Plus there is also the plain old randomess of it all.....