From the app's website:
Fake Follower Check does what it says. It checks your Twitter followers to see if they are real or fake. Or in other words, it checks to see how many spam accounts are following you.
However, the final number for very popular accounts is more a "ballpark" number than anything:
For those of you with 10,000 followers or less we believe our tool will provide a very accurate insight into how many inactive and fake followers you have.
If you're very 'popular' the tool will still provide good insight but may better reflect your current follower activity rather than your whole follower base.
Insane Clown Posse:
Captain Horatio Magellan Crunch:
Surprisingly enough, novelty Twitter accounts fared slightly better: