Remarkably, as short-term interest rates rose, medium- and long-term interest rates did not, something that was referred to as a “conundrum.” One hypothesis is that foreign central banks that were accumulating trillions of dollars finally figured out that they were likely to be holding these reserves for years to come, and could afford to put at least some of the money into medium-term US treasury notes yielding (initially) far higher returns than T-bills.