If you do "emerge --help" you can see in the usage part: emerge --help -h [ rsync | system | config ] so, "emerge -h system" and "emerge -h config" show detailed help about those commands, but "emerge -h rsync" does the same as if "-h" or "--help" wasn't especified.
this is a duplicate of http://bugs.gentoo.org/show_bug.cgi?id=4438
Will be fixed in Portage 2.0.12