iirc, the problem with the .458 Win Mag in its early days was the nitropowders they had then where not realy the right stuff for this "small capacity" case. the powders don't create the power or simple did not burn in the way it was needed to get the proper result. maybe it happen also with long time storage that the powder don't burn anymore like it was when being fresh.
of course, winchester in it's lab with the extra long pressure test barrels get the advertised speed and print it on the box. with modern nitro powder development there must at least one that fits perfect into the 458.

you can build a high reputation in decades that will be lost in a very short time.