One person might have a particular interest in say Windows on the mac and the other in Mac Programming. There might be a need for a moderator in the windows on a mac forums and areas in general and not in the mac programming side of things. Yes mods generally go everywhere but an interest in the area that needs more mod work does have a huge advantage.
This criterion raises and interesting question for me: why?
To be sure moderators each have their own interests and aren't going to spend 100% of their time evenly across each forum and sub-forum, but is that really crucial for selecting the next moderator?
I always imagined that a moderator would address post reports regardless of what forum it comes from, so in that regard wouldn't other criteria take precedence over where the moderator spends more of their time when on the site?
To me it seems more important to see if the candidate has been a positive presence on the forums, follows the rules consistently, helps out whenever possible, etc, because those are the factors that matter most when that moderator makes decisions about post reports.
Just thinking out loud.