Try not to get too excited just yet (easier said than done, right?). Many programs send out interviews in batches...especially the bigger programs. And you don't know the backgrounds of students who post interview invites...Example: a med student at Michigan can probably expect to get an interview invitation to that program earlier than the rest of us.
And remember, everyone gets rejected. Even the best applicants. And sometimes there is no rhyme or reason. You can get an interview to Harvard, but not Penn; Cleveland Clinic, but not Cincinnati; Vandy but not UAB. There is definitely a significant element of chance in the process.