When UTF-8 was first defined, they didn't know how big the Unicode range was going to be, so they defined it as a 1-6 byte encoding that could encode any 32-bit codepoint.
When Unicode was deemed to end at U+10FFFF (because that's the largest value that UTF-16 can encode), UTF-8 was revised to be a 1-4 byte encoding that ends in the same place.
Python clearly implements UTF-8 in a way that uses at most four bytes per codepoint (why support five and six byte sequences if they'll never be used?). I think what we're seeing in '\xfb\x9b\xbb\xaf' is four bytes out of a six byte sequence.
Hmm, may I ask what makes utf8 won't produce U+DEADBEEF? Or something remotely like that?
Edit:
'\xfb\x9b\xbb\xaf'.decode('utf8')
UnicodeDecodeError: 'utf8' codec can't decode byte 0xfb in position 0: invalid start byte