Why is "utf-8" an acceptable encoding when it's not one of the aliases known to the encodings module

Per the docs:

Notice that spelling alternatives that only differ in case or use a hyphen instead of an underscore are also valid aliases because they are equivalent when normalized by normalizeencoding(). For example, 'utf-8' is a valid alias for the 'utf8' codec.

Normalization works as follows: all non-alphanumeric characters except the dot used for Python package names are collapsed and replaced with a single underscore, leading and trailing underscores are removed. For example, ' -;#' becomes ''.

So 'utf-8' is normalized to 'utf8' which is in the aliases mapping:

>>> from encodings import normalizeencoding
>>> from encodings.aliases import aliases
>>> normalizeencoding("utf-8")
'utf8'
>>> normalize_encoding("utf-8") in aliases.values()
True