Mike W is right about processing time. I've had (and heard of) sets sent to me that were months old before I got it. I've turned in sets that didn't close out for a couple of months, too, just due to AD/ED backlog.
I grade them when I have the time. Sometimes the time I thought I would have < the time I actually have, so unfortunately the exams end up lower priority. The bottom line is that for a set to land that quickly, you are fortunate that everyone's time line was in synch. That is great and enjoy it because it doesn't happen very often. :-)