cheapest-to-deliver

Among the T-bonds available for delivery (the short position is given a choice in order to avoid a liquidity squeeze on a single bond), the cheapest to deliver (CTD) bond minimizes the net cost. David's XLS is here: https://www.dropbox.com/s/0145of75vjwhb8c/082218-tbond-ctd.xlsx

In reference to; R19.P1.T3.Hull Study Notes How do we arrive at the conclusion below...am a little stuck on this...any inputs/insights would be much appreciated .. If bond yields are less than 6%, this favors delivery of high-coupon, short-maturity bonds; i.e., bonds with lower durations...