...
Since spammers don't usually highlight the fact that their mail is unwanted, unsolicited ads, SpamAssassin has to try to work it out – and it's not always easy to do. And it's only a computer program, so it does get it wrong sometimes – even humans do.
We use a huge database of mail traffic, and analyse the hit rates of each rule to maintain quality and try to ensure that they only hit spam (as much as possible, at least). Rules that hit non-spam get very low scores.
...