Perhaps I am interacting poorly with the idea of genre as opposed to rules. The way I generally see it, the rules are the rules, and rather than ignoring something because it doesn't fit the flavor, you figure out what twists in logic are required to make it fit. Because what is explicitly stated in the rule books is really the only thing everyone can agree on. (And even then there are arguments.)
So I have to accept the Garou mating chart probability as canon, because the books tell me it is. As for why everyone on Earth isn't a Kin or Garou at this point, well, I have to invent in my mind the loophole of "Once you're far enough away from Garou blood, the spirits start ignoring you and you return to registering as human." Hence the "possibility to change" listed in the Rite would be "making sure you're still Kinfolk."* Sure, this would be rather unnecessary in the cases of high pure breed, but after millions of years mixing with non-Garou the vast majority of Kinfolk are nearly human, or, in some flat-out bizarre cases, are born of people who are both too human to produce a Garou, but just Garou enough together that there's a chance to have a Kinfolk again. This also explains why things like "This incredibly Pure Bred house hasn't had children in a while" is flat out creepy and bizarre, rather than just a mildly unfortunate circumstance of being Garou.
Yes, this is wonky and stupid and should certainly never be published in a book anywhere, but it's a more subtle way to make what's stated make sense than saying "X is so because it's published, but Y isn't so because though it's published, it doesn't make any sense."
Now at this point I've gone way into interpretation and don't expect other interpretations to sync up with mine, but I will continue to assume that published things work the way they're published, and that if something is allowed by those things, even if genre implies that it's rare, it remains a possibility. Because honestly, the edge cases are intriguing and frequently messed up, and so long as they remain rare, I don't see how exploring them takes away from a genre that revels in things that are intriguing and frequently messed up.
*My original interpretation of the Rite, not saying it's correct, just that it's how I read it at the time.