Because the book wasn't made with your rifle. Throat, chamber, internal barrel dia. choice of powder primer and bullets can all affect your pressure and velocity.

I have some loads that are maxed out 1-2 grains below "book", and others that are maxed out a few grains above. I can swing my velocities 200 fps just by changing primers and brass, and in some instances adjusted my powder charges accordingly.

Last edited by antelope_sniper; 09/01/15.
You didn't use logic or reason to get into this opinion, I cannot use logic or reason to get you out of it.

You cannot over estimate the unimportance of nearly everything. John Maxwell