Well, in Normal Again sense nothing from season 1 to the time in season 6 was real, so why should her hair be any different.
I think he was saying that her haircut was the same when she was in the institution as it was when she was in Sunnydale during "Normal Again" since they filmed everything at once.
If they could make Willow have Season 1 hair in "Restless" then why couldn't they make Buffy have Season 1 hair in "Normal Again"?