The riddle of Ueda

[GShock112]

New Tenka Fubu (Daimyo difficulty)
I loaded my save normally and I mistakenly attacked Ueda from Gifu with my bigger army finding out I had 8 total units and the Takeda had 18 units, impossible to win here...

... so I reloaded the game, merged my 2 armies in Gifu and this time attacked again Ueda in full force. Imagine my surprise when I saw the same Takeda army had 25 units. Where did those extra Takeda units come from!?

In this new battle I won and I have both saves if needed.

[rbodleyscott]

New Tenka Fubu (Daimyo difficulty)
I loaded my save normally and I mistakenly attacked Ueda from Gifu with my bigger army finding out I had 8 total units and the Takeda had 18 units, impossible to win here...

... so I reloaded the game, merged my 2 armies in Gifu and this time attacked again Ueda in full force. Imagine my surprise when I saw the same Takeda army had 25 units. Where did those extra Takeda units come from!?

They were auxiliary units that were not called up to join the field army for the first invasion as they were not needed.

See manual, section 19.11.2

19.11.2 Auxiliary Units
In addition to field army units, there are assumed to be additional garrison and local units that will defend a province’s town(s) even in the absence of a field army. This is why it can take several turns to subdue a province after the province owner’s field army has been driven out.

Some of these auxiliary units may also reinforce the local field army if their province is invaded, or if their friends send a field army to relieve them. The maximum points value of auxiliary forces available for field battles is 300 points in an originally-owned province and 150 points in a captured province. They will only be mobilised if the threat is deemed sufficient to warrant this, and the minimum will be mobilised to match the perceived threat.

Provided that an invader invades with a substantial force, the full allowance of auxiliary troops is only available when the province is first invaded (each time this occurs). A counterattack in the province owner’s next turn may be supported by up to half the full number of auxiliary troops. Later in the siege of the province, no auxiliary forces are available because they are all assumed to be penned in by the besieging forces. Small invading forces may be insufficient to suppress the local units in this way.

The current availability of local auxiliary troops (or otherwise) can be seen in the province tooltip.

Note that auxiliary units can assist the field army of the current owner of a province whether that be the AI or the player.

This can be utilised as part of your strategy. Leave one field army unit in a province, and if the enemy invades with a smallish army, the local militia may come out to fight them in battle. Essentially you are getting a free army. It may not be very high quality but if not it will be all the more numerous, giving more opportunities for flanking. And if it loses the battle, only one of your precious field army units will be damaged.

[GShock112]

This is just great.