That's weird. How many charts have you done? IME there is a green square where you O on average and a pink(?) square where your lp ends and af begins on average. Average being the average of the charts you have done on FF, of course.
So... I don't know what the 5 is! I see it though. Perhaps a long time FF user can tell you.
If I ever ovulate I guess I'll have to look for that stat!