How come a Facebook profile says a person has more photos than they do?

For example, on one of my friends' pages it says they have 39 photos of themselves, but when you click on it, it only displays three photos. This is not a new thing to happen, it was the same way with the old profile. I am just curious as to why it would list in the left hand that there were 39 photos, but when you click on it, it only displays three photos and no others.