Users are not unenrolled automatically after enrolment duration

http://docs.moodle.org/21/en/Unenrolment says:

The enrolment duration, which unenrols students after the specified time has elapsed

But in my site, there are lots of students whose enrolment duration has been expired for several months are not unenolled. The enrol method is manual.

And I greped 'enrolperiod', found no code using it to judge whether a user should be unenrolled. Am I mistake?

P.S. My cronjob works well.