How much game time did you pay for between July 1 and August 31? (if you had a sub payment before July 1 whatever time was left on that sub payment counts). You need 90 days of subscription from July 1 forward. If it was less than 90, then if you reach 90 days subscription paid from the July 1 point to September 30, you'll get the mount when the next batch is sent out in October.
90 days subscription time paid for between July 1 - August 31 = Mount sent on Sept 5.
90 days subscription time paid for between July 1 - September 30 = Mount sent on Oct 5.
I think it would have been much nicer if they had waited the 90 day timespan to send things out due to subscription timing and then added a small chunk of time afterwards for a second chance.