In most cases, when the moderating team sees a need for some extra help in a particular forum, community members who show exemplory knowledge in the area in addition to experience being a member of an online forum, are voted into moderator positions by the current moderators on the team. Post count is not one of the priority factors in choosing a moderator.
Different message boards and Forums have their own, individual characteristics and needs, and the qualites which are most important for moderators vary accordingly. Even within a particular online community, needs change over time. When I was first asked to accept a moderator role here, for example, perhaps the most important factor was the need to ensure that each and every topic received reply, and also to ensure that new topics were created to stimulate discussion when things got quiet. As DaniWeb has grown and developed, so has the role of moderators here, and now the role is much more focussed on administrative board functions, for all of our moderators and Admins. Keeping things in order is the primary task we face, because our community here has grown and become more active.
The particular instance you refer to here, I think, is a moderator who was specifically recruited to address the overseeing of a new, specific purpose section. It's a real bonus for our community when that can occur, as it means we gain specific expertise rather than having our 'regulars' address topic areas which fall outside our realms of expertise!