In this case, it looks like you need to do

drush rf

before you do

drush up drupal

This will refresh the list of available updates so Drush knows there is a new release for Drupal.

In this case, it looks like you need to do

drush rf    # pm-refresh

before you do

drush up drupal

This will refresh the list of available updates so Drush knows there is a new release for Drupal.