This has only been allowed into Unicode for compatibility with older encodings and is sort of an exception.
> Multiple Semantics. Because nonspacing combining marks have such a wide variety of applications, they may have multiple semantic values. For example, U+0308 = diaeresis = trema = umlaut = double derivative. Such multiple functions for a single combining mark are not separately encoded in the standard.
[..]
> Occasionally, for compatibility with other standards, a single abstract character may correspond to more than one code point—for example, “Å” corresponds both to U+00C5 Å latin capital letter a with ring above and to U+212B Å angstrom sign.
> Multiple Semantics. Because nonspacing combining marks have such a wide variety of applications, they may have multiple semantic values. For example, U+0308 = diaeresis = trema = umlaut = double derivative. Such multiple functions for a single combining mark are not separately encoded in the standard.
[..]
> Occasionally, for compatibility with other standards, a single abstract character may correspond to more than one code point—for example, “Å” corresponds both to U+00C5 Å latin capital letter a with ring above and to U+212B Å angstrom sign.
Both quotes from Unicode Standard 10.0:
http://www.unicode.org/versions/Unicode10.0.0/UnicodeStandar...