However, in real life, for real humans, over a longer period of time, the drivers with modern automatic transmissions will be more fuel efficient than drivers with similar cars and manual transmissions, because the automatic transmission is a reliable machine that will do the same thing, every time. It doesn't care if you had your morning coffee or not, if some asshole cut you off, or if you got distracted or bored or spaced out and forgot to change your gear to the right one.
I've experienced quite a few humans who, when driving an automatic, seem to select a speed and throttle position that keeps the transmission right at the top of a gear range, with the engine turning much faster than necessary, instead of going slightly faster and letting it upshift. I doubt that's fuel efficient at all.
Even with an automatic the driver has influence on which gear is used, and a driver who naturally doesn't need to know about gearing may unintentionally be telling the transmission, via other inputs such as throttle position and speed, to select too low a gear than what a manual driver would choose.