First of all, good luck on the interview!!
Second, keep in mind that the interview invitations may be issued by AdComs or representatives located in Europe.
Plus, as the invitations are on a rolling basis, it is only natural that most of them would be booked.
Same thing happened to me when I received my interview invitation.
Interesting to mention that my friend who got accepted last year (class of 2015) got a phone call, delivering him the news, in the middle of the night.