首楞嚴義疏注經 4.466

4.466

合具違、順，離但捨受，故云不知。今就知處，違順各四，故得八百。闕於離知，故少四百。

Joining possesses both adverse and agreeable, whereas separation is merely neutral feeling; hence it says "does not know". Now, taking the places of knowing, adverse and agreeable are four hundred each, thus attaining eight hundred. Lacking knowledge in separation, it therefore lacks four hundred.