There has recently been a lot of discussion about the possibility that the Campaign AI is spawning armies to increase the difficulty to the player.
To be clear, the Campaign AI does not spawn additional forces except under the following four circumstances:
The Ashikaga faction. To represent the idea that the Shogun could place a levy upon his subjects and to improve game play the Ashikaga gets some free units.
Rebellions, these troops are the result of a spontaneous (or incited) uprising.
Wako pirates, they are pirates and not subject to normal rules!
European Traders, the Black Ship.
Any other forces you encounter are the result of the AI's recruitment through normal channels.
On harder difficulty levels the AI gets some (minor) recruitment bonuses that may allow it to field armies more quickly than the player.
On easier difficulty levels the AI is penalised and you should be able to out produce it.
Besides normal troop recruitment the AI can also acquire forces through the normal defection and bribery mechanics.
Also note that the AI has a tendency to hide forces and that Ninjas and other agents are not 100% effective in revealing ambushing armies. Don't assume that because an agent can see some location that an army isn't there waiting to ambush.
I hope this clarifies things!
Disclaimer: Any views or opinions expressed here are those of the poster and do not necessarily represent the views or opinions of The Creative Assembly or SEGA.