It has been pointed out to me on this site, although I have not seen it myself, that Austen was no. 51 on the lighthouse wheel, not crossed out, whilst not of course mentioned in the cave. The only numbers not crossed out in the cave are those corresponding to the Numbers. This raises two theories in my mind: 1) The Numbers come from the MIB. This is why he does not concern himself with or is not aware of Kate's candidacy. It would quasi-explain why they are bad luck. 2) Perhaps this means that Kate will ultimately be the replacement for Jacob, or at least the key to the island and the saviour of the Losties, as she appears to have slipped MIB's attention but not Jacob's.
(That is, if The Replacement isn't Xander Harris from Buffy).