We have models about revenue, user growth, user amounts, etc -- all valued per country, etc that we combine to make the estimation. Larger apps make this estimation a lot easier than indie apps, too. You can click through on each app and see more detailed info.
Rovio hasn't been a major player in terms of being a top grossing app recently -- they get good downloads and have brand presence, but in the age of IAP based free to play revenue (24 top grossing apps out of top 25 are free: https://sensortower.com/ios/rankings/top/iphone/us/all-categ...) Rovio's app based model isn't as sticky as long term freemium titles with long term progression. Not to say that Rovio isn't doing great in other parts of it's business!