I wonder if it's wear related. There's (hopefully) a long trend line between new and worn out. As the car wears and the pieces become more intimately acquainted friction may drop as long as they remain properly lubricated. 55 years of experience suggests this doesn't end at 1000 miles. At some point the curve will probably reverse as wear becomes pathological. Yours isn't the first time I've heard of this. I've noticed it in several of my vehicles. Also noticed that on some auto publication long term tests performance at 40k miles exceeds that of post "break-in."
I'm curious. Do you ever use a fuel system cleaner/treatment?
Edit: Just saw the reference to T-Coat's theory. He beat me to it. 180k miles?? Maybe someone's pranking you. They are somehow adding fuel to the car when you're not looking for the fun of watching you go on about how great your car's fuel economy is?
Or, the car may just like you.