The result of last year's Sudanese census (the first since 1956) is highly political, with a direct bearing upon future power and oil sharing. As a result, negotiations over the numbers have been going on for months.
Kiir has previously vowed not to accept anything less than 15 million for Southern Sudan (about a third of the national population). The latest rumours are putting it at 8.2 million (about a fifth of the national population).
This isn't going to be pretty.