That's largely for legacy reasons, but it's also allowed because it's under a different banner.  I know that www.macports.org points to the equivalent of macports.macosforge.org right now, but since "macports.org" is notionally an independent site, there's nothing which disallows the project from establishing its own www.macports.org portal and having two different project faces, one at www.macports.org and one at macports.macosforge.org.  It's only when people are "at macosforge" that they need to see a consistent L&F in order to meet Apple's style guidelines for macosforge.