That's because U+00A0–U+00FF are encoding an earlier character set: "ISO Latin-1" (ISO 8859-1), itself based on DEC's "Multinational Character Set".
The upper/lowercase trick does not apply to ß/ÿ but does in MCS where Ÿ/ÿ are at a different pair of code points.
ISO Latin-1 was the character set on many Unix systems, Amiga OS, MS-Windows (as "Windows-1252" with extra chars), and was for many years the default character set on the web.
It seems to work for codepoints up to U+00FF, for instance:
...but above 0xFF lowercase follows uppercase: Typical for UNICODE though, nothing makes sense ;)