Occasionally, someone who didn't win a slot in the auction still wants a slot very badly and PMs me. In these cases, I sell the 10th slot, and the factoids end up having "0 chance at the moment" for that round. This is not the normal way that things are supposed to work, so the 10th slot is sold at a premium. Recently I've been applying a premium of 20% over the highest (post-flattening) bid in the auction, so 43 mBTC in the most recent case, though somebody bought the 10th slot two rounds in a row, so perhaps I will increase the premium if someone requests the 10th slot again next round.
Can confirm. This, for once, actually happened to me.