Hidden Smart Group Refresh
As of today the recipients are fixed once you schedule the mailing.

You can experiment with https://github.com/3sd/civicrm-recalculate-recipients, if you want the recipients to be determined once the mailing is sent.

As per https://lists.civicrm.org/lists/arc/civicrm-dev/2017-12/msg00012.html there are reasons to prefer both variants.