It probably arises from two sources:
1) Shooters who interpret the higher factory numbers of the Weatherby to mean it's capable of more velocity than the .300 RUM, when in reality the reason is the higher pressure of Wby. factory ammo.
2) Handloaders who feel for some reason the need to defend the .300 Weatherby, and hence load it pretty hot.
Other than that, I dunno. All the laws of internal ballistics say the .300 RUM should be a little faster when loaded to the same pressures.
