I've had the same experience with the LKML lately (been subscribed for about 8 months, generally spam filter has had v. few false positives), can't remember exact numbers, but big, big chunks have been incorrectly labelled.

There seems to be a fair bit of spam sent to the LKML, I don't know whether there's been more lately, but perhaps the large amount of email sent to many people for the LKML and the fact there's a decent amount of real spam sent there, combined with a more aggressive setting is an explanation?