On POSIX, the user scheme has a different 'platlib' location between distutils and sysconfig, dispite the comment claiming they should be the same.

This can be reproduced on Fedora 34's stock Python 3.9:

    $ docker run -it --rm -h=p fedora:34 bash
    ...
    [root@p /]# yum install python3 -y
    ...
    [root@p /]# type python3
    python3 is hashed (/usr/bin/python3)
    [root@p /]# python3 -V
    Python 3.9.6
    [root@p /]# python3.9 -q
    >>> from distutils.command.install import install
    >>> from distutils.dist import Distribution
    >>> c = install(Distribution())
    >>> c.user = True
    >>> c.finalize_options()
    >>> c.install_platlib
    '/root/.local/lib/python3.9/site-packages'
    >>> import sysconfig
    >>> sysconfig.get_path('platlib', 'posix_user')
    '/root/.local/lib64/python3.9/site-packages'

This issue was introduced by the sys.platlibdir value, and its usage in distutils and sysconfig. sysconfig sets posix_user's lib paths like this:

    'purelib': '{userbase}/lib/python{py_version_short}/site-packages',
    'platlib': '{userbase}/{platlibdir}/python{py_version_short}/site-packages',

https://github.com/python/cpython/blob/a40675c659cd8c0699f85ee9ac31660f93f8c2f5/Lib/sysconfig.py#L100-L108

But distutils naively sets both to the same value that does not account for platlibdir:

    'purelib': '$usersite',
    'platlib': '$usersite',

https://github.com/python/cpython/blob/a40675c659cd8c0699f85ee9ac31660f93f8c2f5/Lib/distutils/command/install.py#L68-L87

causing the mismatch, dispite the comment above clearly indicating the values are supposed to be the same.

This was introduced in bpo-1294959 which changed the platlib template to depend on sys.platlibdir, so a mismatch happens when the value of sys.platlibdir is not 'lib'.

(Adding frenzy and vstinner to the nosy list since you introduced the comment in distutils and the sys.platlibdir change, respectively.)