Having worked in rostering, I would venture that this has nothing to do with your planning and bid system. Simply, inasmuch as the schedule of the company keeps changing (flight cancellations, supplementary flights, type change on specific leg...), the later you publish the crew schedule, the less changes you have to make to them...
In the days of Swissair (also with AdOpt PBS system), the head of scheduling was telling me that every day that the crew skeds were published earlier meant about 11'000 (!) individual crew schedule changes more to be processed by crew control afterwards...