De man python :
PYTHONPATH
Augments the default search path for module files. The format
is the same as the shell's $PATH: one or more directory path‐
names separated by colons. Non-existent directories are
silently ignored. The default search path is installation
dependent, but generally begins with ${prefix}/lib/python<ver‐
sion> (see PYTHONHOME above). The default search path is always
appended to $PYTHONPATH. If a script argument is given, the
directory containing the script is inserted in the path in front
of $PYTHONPATH. The search path can be manipulated from within
a Python program as the variable sys.path.
A variável $PYTHONPATH apenas especifica locais adicionais dos quais você pode importar módulos. Não tem nada a ver com a localização do executável do interpretador Python que você obtém como saída de which python .