I finally figured out the formula. Between 28 days and 31 days of a stay, the price is at the rate of the monthly discount. i.e. : monthly rate/31 days x 28 days.
3500 per month discount rate / 31 days = 112.90 daily rate x 28 days = 3161.29
The confusing verbiage of the site seemed to indicate you would expect the FULL monthly discount rate of $3500 after 28 days, which is what confused me.
Hopefully this formula will help others trying to learn the system for the first time.
By the way, do not waste a minute of your time with Chat Support, they do not understand English, they do not read your clearly documented inquiry with supporting screen shots, and they send back ridiculous and irrelevant responses that are of no help whatsoever.
Chat Support always says they will turn the inquiry over to a supervisor, who is never there, and you will get a response back from them "soon". But that no supervisor is at work "that day", 5 different inquiries on different days and different weeks all said this same response.
If you call in to phone support, they will tell you that a supervisor will review the matter, but no supervisors are at work that day, ever (again). They explain that a supervisor will call you back in 48 hours, but no one EVER calls back. Ever. Chat Support does not even understand how the software works at all, every one that responded to me just pasted a link to a help page in the chat and closed it, with no actual reading of my inquiry and no response that was even remotely helpful.
I had to explain this pricing formula to a supervisor, and they still didn't get it. Avoid the frustration of trying to contact customer service in India, they are untrained in customer service, untrained in the operation of the software, and completely ineffectual. Their sole purpose seems to be to frustrate the user into giving up ever asking a question ever again. In that regard they are indeed Super Successful. ABnB would do well to fire that entire division and start over with a new fully trained customer service division from an outsourced company that is properly incentivized to learn the software and proper customer service interactions in order to maintain their contract. All the current group does is post gibberish responses and frustrate the users. This similar experience can be found all over the web.