universal binaries seems difficult

Creating portfiles/Makefiles for universal binary builds seems not so easy. Example: port install zlib -universal produces a fat ppc/i386 library on a ppc machine, while it produces non-fat i386 library on an intel platform. port install freetype -universal produces non-fat i386 libraries on an intel mac, while the installation stops with errors on a ppc mac. port install rxvt -universal produces a non-fat i386 executable on an intel mac and a non-fat ppc executable on a ppc mac. Is this the rule, that the MacPort -universal directive is ignored or has unpredictable results for most ports?