Your Exhaust setup could have a lot to do with it... Somecars have more or less backpressure- determined by pipe diameter, tailpipe type, and muffler type- This will tend to make the car run "easier" at a given speed/rpm zone.
Its not really a problem, more of a cause/effect type thing. Nothing to worry about.
I've heard it referred to as the engine's sweet spot. It's a combination of things that come together for near perfection at a given RPM/Speed that you just know you could drive forever without a problem. Maybe mental jumps into the game to? Enjoy it to its fullest but don't over analyze or it may disappear!