[Attilitus]

There are two possible reasons.

1) When you assign a user to be moderator of a forum it gives you the option to "auto assign" them a usergroup. Perhaps your board is auto-assigning them to the banner usergroup.

2) The legitimate moderator usergroup has improperly set permissions. Check through your usergroup settings to ensure that they are not set up as a banned usergroup.

If that does not solve your problem, post back including any mods you have installed/things that would help us figure out what is the problem.

~Attilitus