It’s not the odd calibers; it’s the difference in the cartridge that separates them apart.
One must note however when comparing the .300 RUM and the .300 Weatherby Magnum the difference in factory loadings. Performance data is often listed on the side of the ammunition box for those who wish to do an in-store comparison. On average the Weatherby cartridge is listed with higher performance. This is because from the factory Weatherby loads its rounds with a greater powder charge than does Remington. This is because Remington generally sells rounds below the cartridges maximum performance envelope so that its recoil will be more forgiving to shooters. In order to take advantage of the greater case capacity of the non-belted Remington round one must handload it to a greater powder charge.

They are called proprietary cartridges. They are largely cartridges designed and developed by Roy Weatherby, founder of the company.

Roy was just an odd sort of fellow.

I read someplace that the round shoulder does not add anything to performance and makes the brass expensive to manufacture. It just differentiates the cartridges from all others. That is a marketing ploy.

Roy Weatherby was a master marketer. He differentiated his cartridges (and guns) in looks, name and performance. Brilliant!