I've never figured that out. When I first open my tree where I am the central person it says 358 of 18173 people. The screen has 8 people on it at that point. How they arrived at 358 I have no idea. Sometimes depending where on the tree I am looking it will have other numbers. Sometimes it won't even have the 18173 number but something else. Like 3 out of 100 or 3 out of 50, or something like that. But it's the same tree with the same number of people in it.
The online tree does weird things. Recently it has started reversing the husband and wife. Everything will be fine untill I add a sibling then the mother switches to the left and father to the right. Family Tree Builder catches this and says that everything is fine except that I have the wrong genders with the parents. All I did was add a sibling and it screwed everything up. In the info on the person online it still says that the spouse is right, it just reversed the places. But it doesn't always do this. It seems to be random.