At equal pressures the .257 Roy will beat the .25-06 AI by around 200 fps, depending on the load. Thus if somebody is stoking the .25-06 AI white-hot and getting 3600 out of a 100-grain bullet, then the same "loading technique" would get 3800+ out if a 100 in the .257 Weatherby. And some people do. It's that simple.
It also isn't magic. It's just pushing the cartridge beyond the standard limits. Whether this is smart is debatable.
