There have been a few cases where I have seen availability on the c x site but when ringing Cx to book I have been told The seat is not available so it is not necessarily cx holding back Seats for its own pax
CX site is inaccurate, so it is possible that availability is different when you call. But as CX DM, with the exception of one or two routes, the MPC agent would ask you which day/flight you would prefer, and your chance of getting that flight is rather high.