Maybe the automatics are just more economical? They have this ability to coast on the overrun which is bound to help, the manual doesn't do this (well ours doesn't anyway). Best I ever got on a long run in early days of ownership was 37Mpg. I've no doubt it could be beaten with a large dollop of self restraint, but life's too short to worry about what MPG's you can get out of a hot hatch.
Mine is a manual gearbox.