Because she was originally in the role that Aerith fills in kh
Dissidia brought us Clerra. KH almost gave us Claya.
Alas, what could have been!
Ryu: On the topic of the "expanded on the story with Cloud" quote, I also don't have access to the Ultimania, but I'm willing to trust this conclusion.
Mako: Decided to reply to you here. While there is a scene where it appears that Squall has heard Tifa speak, I don't think that precludes Tifa being something other than an ordinary human. After all, Sephiroth is the embodiment of Cloud's darkness and definitely not an ordinary human, yet Donald and Goofy can see him.
Naturally, Sora would be able to given who he is -- and presumably their friendship with Sora has something to do with why they can too. Either that, or the proximity of the keyblade.
Whatever the case, what's important is that there's a precedent for those who aren't special snowflakes being able to see embodiments of Cloud's persona. As well, the director said he intended to leave the impression that it was possible Tifa isn't human -- so the part where Squall seems to hear her is either a goof or not relevant to the analysis.
EDIT: hitoshura, I never realized you were the Schism guy! I apologize. I'll update the FAQ to credit you properly.