My theory is that homepages (that are PFI) are definately treated differently. Every other theory I've had (or heard from others) has been proved wrong.
Furthermore...I've never seen evidence of a non-PFI homepage getting phantomed out (while the rest of the site remains)... only PFI home pages. That's just an observation... not proof. But it can be backed up with several examples.
Now... I just don't know "how" or "why" they are treated differently, or what causes them to get phantomed out.
Still working on it.