craigiz1,
I'm almost positive that is what you are seeing. If 1 former member with more WUs completed than you and the "inactivies" in that area, leaves and takes their WUs with them they are removed from the stats and everyone below would be moved up one.
Now you ask why would it show on some and not on others? Well if that same "inactive" who gained one spot from the loss of a member was also passed by someone during the same 24hr period he would have no change, 1gain - 1loss = 0change
The loss of a members WUs can really jumble the standings on many levels, just like having a new member join who brings a large number of WUs completed along with him will cause eveyone below to lose a spot.
Hope that helps.
SkyHook