I was upgraded from Silver to Platinum, despite being 7 nights short of GOLD! No idea why, other than perhaps a large number of stays/nights in the last quarter of the year (as well as getting the top-level credit card and booking a dinner for 75 people at a full-service Marriott). I went right from corporal to lieutenant, skipping right over sergeant!

As others have said, this is smart business by Marriott to encourage the brand loyalty that these programs are all about.