Round table

But not all units grow. If the unit is very small, it might not grow. I once encountered 4 Archdevils in a treasure chest in Pandora. They didn't grow. Apparently, the growth of 0.4 Archdevils per week was rounded up according to the rules of mathematics.

Even small units grow, just not every week. The principle is unknown; it might be random, or it might be a cumulative effect. Try generating a Guardian of Order from Crystal or Rust Dragons: it's unlikely that their numbers will increase in the second week, but they will definitely increase by the 4th or 5th week.
In fixed battles, neutrals may not grow at all if the corresponding option is set in the unit's properties.