I might venture to say that schools also might make early offers to students who demonstrate they really want to attend that school and its their first choice, if the student has acceptable numbers (i.e. at the medians). This is purely speculation but it might make sense from the standpoint that they want to get qualified people in and get as many seats filled as early as possible. Because really, if you're an auto-admit at a school, chances are that you will NOT be attending (and I'm sure adcomms are painfully aware of this).