part of it is math. 27 hours for 400K but only another 13 hours for the next 200k on a 600K. Have to bank a lot time in the first 400k of a 600k to be able to sleep on a 600k. Then on a 1200K, you get 50 hours for the second 600K.
I mentioned it because I spent a lot of time thinking about it for hours on end this weekend waiting for riders to stamp their card, worrying they would not make the times.