The linked Implementation Note says $60B / month of Treasury and $17.5B / month of MBS, with the latter doubling to $35B / month in September. The Implementati

The linked Implementation Note says $60B / month of Treasury and $17.5B / month of MBS, with the latter doubling to $35B / month in September. The Implementation Note says it only applies as principal is paid, so this is an upper bound. According to the maturity distribution at https://www.federalreserve.gov/releases/h41/ https://www.federalreserve.gov/releases/h41/ the Fed has effectively all of its MBS with maturity > 1 year, with the vast majority over 10 years. AFAIK we don't have any other maturity information that "over 10 years", but I might assume it's 20 years on average because it's uniformly distributed between 10-30 years (AFAIK 30 years is typically the max mortgage time in the US). Dividing $2.7 trillion by 240 months gives $11.25 billion per month. Which implies an increase of the cap from $17.5B to $35B is a no-op, as rollover would already be maxed a little over $11B. So why bother? Any bond / Fed experts able to shine a light on what's wrong with my numbers?