Well interestingly ...try this out on your next flight.. ...say you are flying at 75 C.I. look at the progress page(in crz) check out fuel at destination, then change to cost index of 18 and u get an extra like .5-1ton of fuel at destination.
I guess with a low CI, u fly nose up...causing more drag, u fly lower speeds and therefore you are in the air longer but Fuel at Destination is higher....Thats the confusing part...