If they had 14TH/s S9's, why is the Jan 11-20,2018 batch 13.5TH/s? Should I just wait for the next batch? It doesn't seem like this one is selling out as quickly as others have described previous batches of the S9. Somewhere in the 249 pages of this thread, I read that one batch sold out in hours.
The sales rate depends largely on their (expected) stock quantity and the demand. Neither factors are publicly available. At the rate they are pumping out miners between now and January, one would expect the difficulty to escalate greatly. This may also be influencing sales.
This next bit is speculation:
History shows that as Bitmain starts producing miners that have better and better hashrates (also with lower failures) and then suddenly starts selling-off large quantities of batches which seem to be a regression towards earlier models' hashrates, they are dumping their stock to prepare for an improved model.