They pay at every threshold last I know and typically once per block.
Either:
a. random person pointed their 3.x btc payout to LC's address to cause a share price spike and make it up there
b. LC is getting about 3.x btc per block on slush
C. LC had 3.x btc and lowered the threshold to allow a payout of the balance of 3.x btc.
While I would not rule out a, b and c are both promising.
-Ukyo
Option b) has just been ruled out. Slush just solved a block and no payout to Labcoin.
Now a) and c) stay open.
don't know how you worked that out..