If you have a 7 day expiration, some sysadmin is going to set up the renewal check on a weekly cron job. When the cert renewal fails, they don't have any time to act. 6 day expiration = you run the check daily. If something fails to renew, you might have a little more notice.
Or a manual job: renew the certificate every Monday morning after grabbing your coffee.
I could see that value being chosen so people have no incentive to game the cert expiry. For whatever peace of mind it gives them to rotate certs only at low traffic times of the week.