Newmarket Hols
Generally speaking they don't run any departures during the Jul-Aug peak, they tend to stick to early and late season as it means that they can source their hotel rooms and charter flights much more cheaply.
I haven't checked, but it may be that, in addition to this, Newmarket never did have any Sep or Oct departures planned from Manston this year anyway...which might explain why the soonest availability is October.
Someone closer to the airport might be able to confirm if there were any Sep/Oct departures and, if so, this on the other hand might prove the theory that the flights are already sold out..