how to enable autolearn?

[L A Walsh]

I have:

bayes_auto_learn_threshold_nonspam -5.0
bayes_auto_learn_threshold_spam 10.0

in my user_prefs.

When I get a message though, I see autolearn being set to 'no':

X-Spam-Status: Yes, score=18.7 required=4.8 tests=BAYES_99,BAYES_999,
 FORGED_HOTMAIL_RCVD2,FREEMAIL_FROM,FREEMAIL_REPLYTO_END_DIGIT,
 LOCALPART_IN_SUBJECT,TVD_SPACE_RATIO,TVD_SPACE_RATIO_MINFP,
 USER_IN_BLACKLIST_TO autolearn=no autolearn_force=no version=3.4.0

Shouldn't a score of 18.7 trigger an autolearn?

Thanks!