For me this is really a key question and one of the mysteries for me.
Somebody is supplying (virtually) unlimited borrow. Who is the primary supplier and who is the primary borrower and why at such a low rate? At this point, you would think the primary supplier would want to jam the primary borrower with higher and higher rates (because they could), which their position (of the borrower) would force them to pay.
So why leave money on the table?
Unless of course the supplier and borrower were possibly working together somehow?
Don't know the answer, and will just have to wait and see.