Well, if they don't say why they say that we really don't know.
Perhaps because real "magnums" heat barrels quickly, few shooters give sufficent time for cooling between shots so the barrels warp and that blows groups.
I shot 60 rounds of full boogie 7mm Mag one day from a rest. OUCH! I was able to use my shoulder again after a few days, and the bruise went away after a couple of weeks.