In my experience, getting modded is more about getting in the conversation early. Probably few moderators come back to topics, so posting a day later makes for very few mods. That is if they are will to read deep into the comment.
Maybe the change in the number of mods has to do with more posts or how quick you are to read a story. I don't know the formula of number of moderators either. Lots of factors could play into it, including "dying" depending on what you think it means for it to die.