US Consulate visa interview appointment scheduling

TL;DR - Do US consulates "release" visa interview slots on a somewhat ad-hoc basis making it necessary to regularly check their web site in order to make a reservation for an interview a few weeks ahead of time?

I am presently in the USA on an H-1B visa along with my wife with an H-4.
We are planning to travel back to my home country (South Africa) over the Dec'16/Jan'17 holiday period and, since our original visas expired (but have ofc been renewed) we're going to have to pay a visit to a US consulate in SA (preferably Cape Town) in order to have new visas issued so that we can return to the USA in January.

When making our travel plans I checked the visa interview availability on the US State Dept web site for the consulates in SA and saw that appointments were open to be booked 8 or 9 calendar days in advance (and processing time should be 2 working days). So I finalized my plans and then proceeded to complete our online DS160s and continued through to book an appointment. After paying the MRV fee and getting to the booking calendar, I found that I could book a slot almost any day in November, but the only day open in December was the 6th (10 days before I planned to arrive).
So I called the US Consulate visa call-center in SA and was advised that the consulate had not yet released their full availability for December and that I should book a slot on the 6th, but check back daily and reschedule when new dates became available,

Does anyone know if this really happens?
Is there any real likelihood that I will see some appointment dates open up later in the month if I wait a few days or should I cut my losses now and make other arrangements ... ?