Joshua, people just don't bother going online and releasing any slots, since there is no incentive to do that. IF rec.gov would give a small refund, they'd definitely make a profit, because the slots could then be sold again. But I am told that giving money back is a crazy big deal. ...you know, all those online stores never do it (not)
The only true indicator you can use is the unused permits pages. And also, watch for returned slots online in the last two weeks before you go. That's when a few people go online and confirm their reservations, and some will return some or all their slots (but it's just rare).