No attempt to make you look like a fool, Miller. Simply posting the current numbers for British and American shot sizes as listed in current sources (which are different than what is shown in the Hallowell chart you posted).
No question that Brit 6 is closer to US 7 than any of our other shot sizes. But it seems to me quite possible that someone selecting a load for hunting a particular game bird might want to know about the 11% difference . . . which equates to a pattern that's either 11% denser or 11% less dense, depending on which you pick. I've used factory loads of both Brit 6's and American 7's in the open barrel of my guns for pheasants. I prefer a less dense pattern, so I like the Brit 6's. (Might end up with a pellet or two less in the meat.) Late season grouse, or perhaps quail in the tight barrel of a double . . . I'd take the ounce of 7's. Smaller birds, denser pattern. Not a whole lot different than choosing between US 7's and 7 1/2's. Nice to have the 7's available if I want a pattern that's less dense than 7 1/2's (and slightly larger pellets) without dropping all the way down to 6's.
Too bad that both US 7's and Brit 6's are hard to find, in terms of finding them off the shelf. They're both good choices in that relatively large gap between US 6's and 7 1/2's. I'm glad that I can find both on the shelves in my gun room.
