I think I would interpret the saying in more black and white terms, instead of gradations like 90% versus 98%. Like pass/fail.

Having none of some critical item is obviously a fail. Having just one item is either a pass or fail, depending on which side of Murphy's Law you happen to fall on in any given situation. But if a critical "fail" can be avoided by having a backup, then it seems only logical to do that.

Of course, it is overkill to apply the saying to every single item in your kit, and there, I wholeheartedly agree that the "math" doesn't add up. And having five of an item is not necessarily any more helpful than two of something, or perhaps not even necessary at all. I may survive some situation just fine without a bandana, let alone five bandanas, say.