I can confirm now that the units are *not* necessarily of the same type.
Long version: I had a quest to recruit 5 units from my era or 7 from the previous era. Before collecting from my traz, I had 1/7. After collecting from my traz I had 4/7, even though my traz produces 5 units. So it probably gave me 3 from my previous era and 2 from my era.