Not sure what happened on the 17th. The tracking of user movement is not perfect for some of the non boinc projects though.
User ranking is a simple one though. I don't 'hard' delete users which are gone (often name changes), I simply mark them for deletion. They are not used as part of the project ranking at all and in some places I don't actually show them, but by no means all places. I've deleted them now, so next time it runs they won't be visible.