Two things.
1) Members who have not cookied their login initially hit the page as a guest.
2) Web crawlers (site indexers) are guests
Lastly, I think there are some bugs with this, I think when it is 75 that it is exaggerated. I have IP traffic stats and I can tell you that we do not have 75 unique IP visits throughout the whole day. It is interesting to watch that, but I don't count it as a real statistic, but there is some hints of truth to it.
