Nevermind, I think I understand now that's the hidden nationality mechanism.
In general, the Civ3-AI
loves the Hidden Nationality unit-ability, and may/will often build those units to the exclusion of better ones.
For example, in the Sengoku scenario, the AI will often spam Ninjas (A.D.M=6.2.1, requires Jade, unit-abilities HN, All Terrain As Roads, Amphibious, Stealth-Attack, Invisible) in preference over stronger attackers such as Mounted Samurai (A.D.M=7.7.2, requires Horses + Iron, Blitz), Samurai Archers (A.D.M=6.3.1, B.R.F=3.1.1, no resources, Amphibious), and even Samurai Arqebusiers (A.D.M=10.8.1, B.R.F=8.3.2 [IIRC], requires Saltpeter, Amphibious).
And then, instead of assigning its Ninja to their primary purpose of infiltrating and assassinating opponents' Daimyo (the Scenario uses the Regicide option) — and despite the fact that the Civ3 engine allows AI-Civs (but not the Human) to use HN-units to attack/capture opponents' cities during peacetime — it will tend to use them mainly to kidnap its opponents'
Workers