Since iOS was big before Android was, it stands to reason that many apps would have been developed for iOS and then not ported to Android. Whereas now Android is a huge market that most will want to develop for, but they're not going to stop developing for iOS - so I would expect a vast majority of apps to be on both platforms.
Since there was never a time when Android had a large market but iOS didn', it not surprising that the only Android-exclusive Apps are ones that can't be done on iOS.
That seems reasonable, since the idea of a "killer app" could be defined as one whose existence leads you to pick the platform it's on over any other.
If an app is available on both Android and iPhone, then it's not decisive. You can still choose either platform.
> A final thought, regarding Android’s relative weakness as a software platform. iOS’s exclusivity for a bunch of big-name mobile games — Need for Speed Undercover, Star Wars: Battle for Hoth, Monopoly, Tetris, The Sims, Assassin’s Creed — has been broken. Not by Android, where none of these games exist, but by Windows Phone 7, a one-month-old platform.
EDIT: Unity will likely make a big difference to the Android appsphere: http://unity3d.com/unity/coming-soon/android
Number does not equal app sales.
> where are the killer games for OS X?