Drush pm-update strange behavior
The thing is, every Drupal site has a variable, in {variable} table, called update_last_check that stores the last time the site had a update check. This variable is mostly update by Drupal cron. It happens that, in some of my sites, this date is too small to need a new check, so Drush skipped it and considered the site up to date. So, in other words, if you want to ensure there's a new update, you should first run a drush cron and then run a drush pm-update.