just out of interest, but why do u need a permissions mask for admins group? (w00t)

Because it's easier to have one than to put hard-coded exceptions all over the place, and because, likely to be used or not, it gives admins full control over permissions, even to the extent of hiding something from themselves should they really want to.

Just off the top of my head, one reason I've seen is some forums have a redirect forum at the very top of their forum lists that redirects to the registration form. This is only shown to guests, and so, of course, you would not want admins to see it.
Some people also hide their trash can as they have no interest in it unless they actually need to get something out of it.

Or perhaps you created a forum that was open a while, but decide to just hide it (but not delete it or it's contents) during a forum reorganization.

There are many reasons to be able to hide forums from yourself. Sometimes you want to keep the data but not have to see it on every login.