annoying to be quite honest.

[Centurion1]

I have found quite the annoying little development. it seems that armies can be spawned by enemy willy nilly and with extreme vetreancy. in my chokosabe campaign (only hard) i am faced with a kono full stack with triple chevroned enemies. this is really annoying. im all for pumping up the enemy to some extent but i never like when the ai cheats.

[Phog_of_War]

I noticed this as well just last night. It also seems to happen only in the very begining of Campaigns, and most of these veteran stacks are from minor Clans.

[Zarky]

I don't think it's really cheating. I've seen enemy stacks have some oddly experienced units, but never a full stack. I think they're just exploiting the benefits of dilemmas or something that gives experience to recruits.

[antisocialmunky]

There's a message that pops up that gives you like 3 stacks of extra units if S2TW detects you've failed the Turing Test.

[Satsuma]

I think that some factions can start, or develop early on, some of the more advanced buildings that provide experience to units.
I've taken a Bessho province that had a Master Bujutsu dojo which provides (I think) +3 exp to units recruited.

It could also be a programmed thing to give the minor factions a better chance of survival in the early game and to prevent blitzing from the player.