I really do not believe you are getting anywhere near an actual 64mpg. I owned the same model and same year bike and even riding it as gently as possible in a high gear on a flat road with no panniers or headwind (and also the 2 previous 1200GSAs I’ve had) - I’ve never seen more than about 54mpg. The moment speeds increase, you put panniers on or you hit traffic you’re looking at high 40s at best. It’s just not that efficient as an engine, even running as lean as it does.
As i said earlier my average is 53mpg, touring two up fully loaded I don't start looking for fuel before 200mls, If I ran till empty I would get 250mls from a tank full.