Just because a school has 100% of it's interview spots open does not imply it is willing to "waste" them just because there are few complete applications. Schools know, within +/- 500-1000, how many applications to expect, and to a great extent what MCAT and GPA scores to expect as well. A school is not going to be fooled and freaked out by a wave of underqualified applicants (believing they offer a perfect cross-section of that year's applicant pool), and throw interviews at anyone that rushes to apply.
The people who apply early tend to be those on SDN- diligent, neurotic, and with very high stats. Those people would get interviews any time of the year anyway, which is a possible explanation for why a higher percentage of interviews are offered very early in the season. like I said, there is absolutely no information available on how many interviews are granted per applicants (or rather, per qualified applicants) at different times of the season, so for now its all speculation.