Usually how it works is that they send the majority of interview invites in December and early January, and then after some people decline interviews (scheduling problems, lack of interest, etc), they send out final interview invites by February for most programs. Then they start sending out admissions offers, and once they've got a good sense of how many people are actually interested in attending, they send out final offers to a handful of people who would have originally been on the fence.
Hence why some people get in without an interview.