I do not think schools "save" the seats. Some schools traditionally start processing appl later thus offering only partially seats on Dec 1. BU and Tufts come to mind. Some schools like Nova, USC, Midwestern and NYU are high priced schools so the acceptance from applicants are lower as many students choose to go to instate less priced schools so they have to give more interviews post Dec to fill up class. Schools like Case have scheduled interviews that go well into April but those schedules are given now so you could be early batch applicant and receive your invite for March now...
Some schools like Marquette pretty much done by Dec 1. Last year they send 81 acceptance on Dec 1 for 80 seats and only few people did not accept. So for Marquette late applicants almost have no chance.
Instate seats are usually filled early for instate schools and there is little movement later as opposed for OOS seats for same school as tuition difference often significant.