It's actually very hard (at least or even) for humans to categorize twitter users as spammer/fake or real.
Re:Fake Twitter followers
by hayksaakian on Wednesday April 10, 2013 @12:25PM
(#5526139)
I would assume tweeting spam is weighed heavily. I doubt they care about simply inactive users.
[quote]
> They use a number of different criteria to identify fake Twitter accounts, including:
> The account is following less than 50 people and has less than one follower
> More than 30% of all tweets use spam phrases, such as “diet,” “make money,” and “work from home”
> The account is more than two months old and still has a default profile image
[/quote]
I don't know exactly how these things were weighted to decide that an account is fake... but apart from the second point, it sounds like "Fake" account could be more like "I signed up just to follow this guy and then forgot about it" account.