Note that the description of the POSIX mkdir utility (*) has something a bit more complex to say about the `-p` option. Instead of simply applying the default umask, it computes """(S_IWUSR|S_IXUSR|~filemask)&0777 as the mode argument, where filemask is the file mode creation mask of the process (see XSH umask)""".

But unless the umask has a pathological value (such as 0o333), it doesn't really matter. The main point is that the original mode argument is ignored.

(*) http://pubs.opengroup.org/onlinepubs/9699919799/utilities/mkdir.html