I believe the Adcom generally try to get the invites to those who need to get plane tickets ASAP, but due to the random nature of when your app is fully read and evaluated, sometimes people will get last minute invites (like, 2 days before SS). The thing is, Super Saturday is OPTIONAL. While I strongly recommend and urge anyone who can to attend Super Saturday (it's a fun and educational 1/2 day event), due to work schedules, ticket prices, and other reasons, you are also given the option to interview locally with an alum.
There is NO difference whether you interview with a student or an alum. In fact, during Super Saturday, you may interview with a student, alum, or even an Adcom (if you're "lucky"

). All interviews are considered with equal weight, so don't panic if you can't make SS.
Hope that helps!