Nice resource hack you got going there?
Just kidding. Anyways, I believe it happens if you delete/rebuild your map database (it removes all the saved rankings you have for maps), then play the map again and get a different ranking on it. For some reason, osu! no longer instantly redownloads your rankings from the server, so it can't keep proper track of your rankings until it redownloads them (which I don't know what triggers it to).