Really? I didnt know that. In the same way I don't quite understand why manual cars have slightly better economy than automatics... The only reason I can think of is that the car doesnt really know what is ahead and changes up and down when maybe we (humans) wouldnt...?
Cruise will not have as light a "foot" as you. It maintains speed up hill and down dale, whereas you can anticipate and have a considerably lighter touch. But it does ache on a long run!