See, this comes from a fundamental misunderstanding concerning "qualification." Indeed, more qualified applicants who apply early are given priority when it comes to interview invitations. That being said, you might have below median statistics and still be one of the more qualified applicants at a school if you have ECs that they find valuable (I don't like saying that there are good and bad ECs, but rather ECs that different schools will apply a certain value to) and also fit the mission of the medical school. Your essays are also very important - I know I had at least one interviewer remark about my personal statement and supplemental essays being both passionate and well written.
Yes, there is no denying that your numbers are one of the most important aspects to your application, but be confident - you receiving an interview means that they find your academic transcript to be strong enough that you could succeed academically at their school, or else they wouldn't have offered the interview.
Now I'll get off my soapbox for a second to answer your question. First, does this school have rolling admissions? If so, then interviewing early is a huge advantage, even if you don't know that the school is extremely selective for interviews. Even if it isn't rolling, they had to make some serious cut downs between primary/secondary applications and interviews, and the fact that you weren't even put on hold to review your application in the future indicates that you were probably in a good to strong position before you even went to the interview. From there, it's up to your interview to REALLY set you apart and earn you an acceptance versus a hold/waitlist spot in October.
