1. Not made by Google or built-in
2. Popular and well made (no long tail)
3. Don't take advantage of things Android can do but iPhone can't (home screens, notifications, deep integration, background services, etc.)
Suppose you had such an app, or were developing one. While Android has passed iOS in new phones sold, it's still catching up on existing userbase. Why wouldn't you have an iPhone version too, since by definition your app already would run fine there?