It seems I received a partial payment of my total due balance. Didn't lose the unpaid portion, it stayed in the total due. Never noticed this before...anyone?
I would guess it's because your paout priority made it so you were the last one the mined block, and the full payout didn't fit in that block, so it payed what it could. But what was left unpaid was under your payment treshold, so you were no longer in the payout queue.