Matt, I'm not sure what happened.  I remove everything under
/mail/grey each night, so perhaps your system tried to call once on
the 7th and then not again until the 13th?  The greylist code does
assume that an unknown caller will call back within the same day.  If
your cron is only running the mail queues every day or week, I'd
suggest changing it to once per hour.  Transient failures are possible
even when calling sites that don't implement greylists.