To the above: H&H had a double chambering they came out with a century or so ago called the .465 nitro or something similar. This bore diameter for the .465 H&H will have been chosen for nostalgia's sake, this being the bore diameter of H&H in a previous cartridge. The name looks and sounds nice too.

As for belt defeating the purpose? Again, nostalgia. The belt on a .375, is it needed? It was to control headspace on those long, sloping shoulders - exactly what it does here as well. Let's face it, the .465 H&H has almost no shoulder, more a couple of tapers.