If a manual enrolment method has a set default duration and users are enrolled via the ajax-y interface, the enrollment duration is not set in the enrolment options (however it is set in the non-javascript enrolment screen). It's set to the default Unlimited duration.

Steps to replicate:
*/ Set the default duration on a manual enrolment method to say, 120 days.
*/ View the enrol users page for that course: /enrol/users.php?id=<id>
*/ Wait for the page to load and then click enrol, in the JS interface, expand the 'Enrolment options' section.
*/ You'll see the duration is set to Unlimited instead of 120.

*/ If you visit the non-js interface (which you can get to by clicking the 'enrol users' button before the JS has a chance to load) you'll see the 'Enrolment duration' field is filled in and is obeyed when you enrol users with that interface.