I imagine two things are at play (although, I'm new to the admissions thing so feel free to correct me).
1.) This forum (and the results page) are not a representative sample of all the people who apply to grad school. The shoe-in candidates who aren't worried about getting in probably don't check this forum or post, (and thus we would never know if they got admission early in January).
2.) The early bird gets the worm. Admissions committees outside of the top 15 want good candidates too, early offers = higher likely hood they get those candidates to sign on.