The significance of the quoted numbers is that they are all encompassing. i.e. if you want to go to war against country A, then you had better take an air group of N F35Cs or N+Y F35Bs.
Y is due to the fact that if you want to hit deep targets - errr, well you can't so the war will last longer. If the Flankers come up to meet you then the B gives you a few 'pump and recommits' the C gives you a whole load more. The B might well have a lower availability rate...hence we arrive at Y.