Luky, Consider that when you open the throttle on a fixed pitch prop, stationary on the ground, the blades are fully stalled - they are churning up the air, while hardly creating thrust. As the plane accelerates, the blade angle more and more matches the angle of the air entering the prop disc. This is like the plane recovering from a stall.
If you think of how much power it takes to fly slow flight right near the stall, compared to how fast the plane will fly with the wing not at all stalled, it's about the same thing. If you've got the prop blades in slow flight, nearly stalled (or fully so) it will take lots of power, so you won't get the static RPM. Once the prop, or the wing recovers, and is less stalled, the drag reduces, and it will speed up on the same power...