i think you're just learning to enjoy and drive the car.
if you haven't noticed, the 6 cyl. has a bit of zoom. it will go up or down as you drive.
it will go up as time goes along.
but until the engine is broken in to your driver's style, it is.....well what it is.
it's within specs for a well-driven 535 without beign scared of the throttle. as you log cars on, mpg should improve.
around town should improve closer to 18-19mpg. on the high way around 70-80 mpg is should head into the 28 mpg area.
i used to the 550xi.
around town it's 23-15.5 maybe 16. on the highway it could hit as far as 25-26----depending on traffic, driving style, snow.
my advice: just enjoy the car!