Not 100% sure but my here is my theory:
CX have two different 777 versions they typically send to Sydney, one has 268 Economy seats (77G 3-3-3 config) and the other has 296 economy seats (77K 3-4-3 config). They often a day or two change the aircraft in the system depending on load - so whilst you might have had the 296 seats loaded in (despite still seeing the 3-3-3 seat map), they have changed this to the 3-3-3 aircraft reducing the number of seats by 28.
Lately, they've been running the 77G with the 3-3-3 config on CX100/101 but they could swap that out last minute with the larger one at the last minute but in a worst case scenario they've got 28 overbookings which they could either upgrade (if space left in higher cabins) or move to a different CX, VA or QF service.