You have the CORRECT ANSWER AND THE SOLVE TO THIS MYSTERY!
There is one additional thing I figured out. The result is a combined discount. Here are how it worked with my numbers....
My nights were $200 per night so a month should be around 6k. Then about a week ago I selected a suggestion box from Airbnb that said something like "welcome long term stays for more visibility" or similar.
I then put in monthly discounted rates for entire months. I put in $4200 if booked for the 30 days.
Then I got a few booking requests where they had received quotes of about $3700 per month. This is what led me to this thread.
My regular monthly discount that I always have turned on is 10%, so what airbnb was doing was taking my $4200 per month and applying that to anything over 28 days and then applying my monthly 10% discount.
So I ended up with a combined discount. One that is obvious (10%) in the monthly discount section of "pricing and availability" on top of the less obvious discount of $4200.
I tested this by eliminating every monthly price I had set and kept the 10% in place.
I then went to a private tab, found my listing and selected 28 days and my discount was only 10%!! So removing those worked!!!