It is because KA900 is the first schedule flight from HKG to PEK, so it always overbook and full load (a lot of group to PEK choosing this earliest flight).
P.S. if a day i start work at 0645 then i will absolutely be one of the handling agents of 900 AND 991. The 900 is one of toughest to handle because it normally has 30+ PNN, but the record is around 100+!!