"Active" doesn't mean quite what
you're assuming it does, though. An active
arbitrator is merely one that's participated *in some way* in the recent
past; it doesn't constrain them to a particular level of activity.
Well, yes, I'm using a slight different meaning of "active". If each
arbitrator isn't doing as many cases, it is reasonable to expect a
higher level of activity in those they do take part it.