Intrigued by the info James imparted here earlier in this thread, I decided to run my own test. I did this by temporarily relisting an old deactivated listing in Firefox browser.
I then tried switching the advance-booking period between three and six months into the future.
I did not at any time manually alter the blocked periods on the calendar.
I took screenshots with two sets of images of the calendar for the months September through December. One set was with the advance-booking setting fixed at six months; the other at three months.
For the month of November, some days that are blocked in one image are unblocked in the other image.
This now confirms beyond any reasonable doubt that some of our blocked days in November were unblocked by a bug in the system.
Unless someone has another explanation?