Overall, Games, and Applications categories are what we call Umbrella Categories. This means that they are a roll up of other smaller categories. For instance, if you were to look at a slot machine game, this app’s hierarchy would look something like this:
Casino > Games > Overall
So really good apps can be ranked in up to 3 categories. For instance, there are a lot of Casino apps which are ranked in the Casino category but NOT ranked in Games or in Overall. This is because there might be 90,000 total casino games, but 1,000,000 total games, and 2,000,000 overall apps. So as the pool gets bigger it becomes harder and harder to get ranked. So only the very very top casino apps would also be ranked in Games and Overall.