The numbers are a code for whatever system the island adheres to
It's been calling out for decades for these numbers
The island doesn't recognise the losties.
It recognises where they are placed
Was Sun in right seat when they made their return flight?
Did the seats decide the numbers or did the pre-existing significance of the number decide the seats.
Why are these six so important?
The island has its own way of reclaiming those who are lost.
Maybe its repeated calls for the significant passengers of that flight have been echoing down the ages, misinterpreted, by those who have heard them. Is their choice of seat significant or was their choice already made for them?