First, the manual IS correct. What they print is exactly what they got.
Second, the major difference between what they got and what you can get is what they shot it in. If they used an instrumented test barrel (to get pressure readings as well as velocity) then that barrel was cut to absolute minimum dimensions. Because it is tighter in all respects, it delivered higher pressures AND higher velocities than a looser sporting barrel will. That's for obvious safety reasons.
So a chronograph reading from your rifle and using your components that registers 100 fps or so less than the "book" number is probably dead smack on.
Cleverly disguised as a responsible adult.