The --help output states that -p is default for superuser, the man page does not. This is very confusing and should be fixed.
The --help output:
-p, --preserve-permissions, --same-permissions
extract information about file permissions
(default for superuser)
--preserve same as both -p and -s
--same-owner try extracting files with the same ownership as
exists in the archive (default for superuser)
The man tar output:
extract all protection information
create extracted files with the same ownership
The tar man page is packaged separately, as the tar tarball does not come with a man page, so this isn't an upstream bug.
Created attachment 301353 [details, diff]
patch adding "default for superuser" to man page
Patch for /usr/portage/app-arch/tar/files/tar.1
man page of >=tar-1.27 contains that piece of information. As we are already in the process of stabilizing =app-arch/tar-1.27.1-r1 let's wait for that version to become stable.