A couple of things that may shed some light: 1)Some boxes of the "Heavy Magnum" don't produce claims. Some well respected posters on a couple of other forums report MVs of less than 2100 fps.

and 2)Because of my interest in the 458WM, and writing a hunting journal on it for North America, I contacted Hornady and Barnes on several occasions. There is a powder that'll produce the claimed MV - AA2230 - to be specific, in 24" tubes. But Hornady recommends H335 as the best overall powder, and that's what I use in my CZ for heavy bullets (500s), getting 2200 fps even because of the very long action and long throat. I can get 2300 fps from my rifle quite easily, but 2200 fps is enough. When I asked Hornady about AA2230, the response was "yes" it will produce the MV listed in their manual from a 24" tube with "certain lots"! Other lots, "No". And, at times certain powders are unavailable (or lots of powder)to them.

Regarding the less than advertised MVs, I suspect, with good reason, that they got a supply of powder that didn't meet specs! That happens. They can't return it if it falls within certain parameters, even a slow lot.