In one of the Fantastical Beasts movies there is mention of atomic bombs in some visions and they see it as a major threat. So it's kinda canon that some non-muggles know about nukes.
That still doesn't give us a reason why customers have this special calculated value. You could just multiply the volume by this percentage to get the absolute value. Why is the percentage preferred over the volume? I see no reason for either side.
Think of it with fewer digits.
If we have 0.999 and 0.111, then yeah, each 1 neatly complements each 9. But you have to consider that each of them result in a 1 the next digit upwards (as 09 + 01 = 10). So 0.999 + 0.111 equals 1.110. What you actually want here is 0.001, which just complements the last 9, which returns a 1 for the next 9, which returns a 1 for the next 9 and so on. 0.999 + 0.001 = 1.000. If you increase the digits, the zeros between the decimal point and the one increase too, causing the number to get smaller and smaller. If continue this infinitely long, the number becomes 0.
Tbf, trans women are only a small percentage of all athletes in the women's category. So it's not so weird that none has won yet.