Scores for all the tests are based on data from mass checks and a required_score of 5. The default required_score in SA installs is 5. That makes sense.
http://spamassassin.apache.org/full/3.3.x/doc/Mail_SpamAssassin_Conf.html recommends against using a required_score of 5 for anything but "a single- user setup". That doesn't make sense. "...if you're an ISP installing SpamAssassin, you should probably set the default to be more conservative, like 8.0 or 10.0." Is there a reason there aren't separate sets of scores generated with lower false positives, instead of recommending higher required_scores?
