Factory loads are usually well below sami safe loadings. What they do is add a little powder just keeping closer to the upper limit but still in safe range. In order to market to hunting types the word "magnum" is easier to sell than the +p and +p+ they use to market pistol ammo to the mall ninja types.
Yeah there is a minute benefit but the practical benefit over a srandard load is pretty close to zero.
Its a good marketing strategy
"Gun control: The theory that a woman found dead in an alley, raped and strangled with her panty hose, is somehow morally superior to a woman explaining to police how her attacker got that fatal bullet wound." — L. Neil Smith