Putting the opening contents of Speculation + the "Contradiction" section here, so when I go through and change it, I'm not just obliterating someone's ideas, and if they want to argue for them, here they are:
The term Commander is used in two separate ways. All units with Leadership, including Warlords, and Chief Warlords, are periodically called Commander by their associates as a title. The actual rank of Commander indicates that a unit has the Leadership Natural Ability, but no confers no Leadership Bonus to units under its command; whereas, Warlords and Chief Warlords confer a Leadership Bonus to units in their stack and in other cases as well.
Contradiction in Terminology
It seems that "Commander" is a more general term - Commanders have an ability to lead other units in combat (not necessarily deriving from Leadership natural ability) while Warlords are Commanders who have Leadership ability and thus give units under their command a Leadership bonus. It should be noted that in some special cases Casters can give a bonus to certain units under their command (known examples are Golems led by a Dirtamancer and uncroaked led by a Croakamancer)Erf-b1-p084a.
- Commander I. Heartly Noah June 3 2009