Here's another oddity to ponder.
I've been monitoring a list of about two dozen phrases for my site for years. Several of them have been #2 or #3 without moving. I looked at those phrases (they're in the same spots), and GWT shows visits from those phrases down anywhere from 45-60% from last year. The pages haven't moved in the SERPS (I've checked multiple datacenters), but the visits are down.
Traffic to my site is about 80% of what it was last year. What I don't understand is how traffic for these particular pages could be down 45-60% when the pages are in the same spots.
BTW, one of the garbage sites I mentioned earlier that had climbed to the number one spot for a phrase not related to my niche has been dropping in rank a bit each day. Today it's gone. Maybe there's a sliver of hope there.