This is just a quick theroy based on what we've seen from the numbers in the lighthouse.
I dont recall seeing desmond HUME on the list, which makes me beliveve that maybe this is why he is special. maybe he is special because he doesnt have a number? he wasnt brought to the island by jacob, and thats why, as Daniel Faraday once said,: 'the rules dont apply to you desmond'
This could also be the case for walt, assuming that the dawson on the lighthouse list is micheal, then maybe walt is special aswell because he is not on the list?