Probably because it barely goes over 20 bits. 20.09ish
If you want to be cheeky you can argue that "hidden assumptions going from a 16 bit wide character set to a 20 bit wide character set" is still correct, because those hidden assumptions are part of UTF-16, and UTF-16 uses exactly 20 bits to express the astral planes, encoding them in a different way from the BMP.
If you want to be cheeky you can argue that "hidden assumptions going from a 16 bit wide character set to a 20 bit wide character set" is still correct, because those hidden assumptions are part of UTF-16, and UTF-16 uses exactly 20 bits to express the astral planes, encoding them in a different way from the BMP.