Keltham gives those words some time to settle; he's kind of guessing that nobody in Golarion has ever before considered the notion of incentive conditions under which stuff will mean stuff.
"So now, of course, we have the problem of finding any scoring rule which has this lovely and desirable property."
"Well, and we'd also like it to have the property that, when something happens, the higher the number you assign to it, the higher your score; or, when it doesn't happen, lower numbers get higher scores."
"So long as we're making up a wishlist, we'd like the method to still work if somebody says '17.3 out of 100'."
"We can take advantage of a symmetry which is that, if something happens 47 out of 100 times, that means it doesn't happen 53 out of 100 times. So the yes-score of 47 should equal the no-score of 53. Or to put it another way, the no-score of 30 is just the yes-score of 70; we don't need separate yes-score and no-score rules."
"And somebody named an important final condition earlier, does anybody happen to remember it?"