When do interview invites end?

It seems like pretty much all programs have sent invitations out already and for the vast majority of them, the dates are already filled. Some chart I saw in charting outcomes said that 20% of invitations come in November and even some are in December. How is this possible if most slots are already filled?

Source: Student Doctor Network - October 28, 2017 Category: Universities & Medical Training Authors: osprey099 Source Type: forums