Batch update: triggered confirmation email
In my CiviCRM experience, this does indeed happen. CiviCRM seems to expect participants to be Registered before they are Attended. If you try to skip this step in the bulk update, it will move them to registered and trigger the email. Have a look at those participants and see if their status is now Registered. 

We ensure that the are moved from the pending status to registered prior to the event as this also ensures that they get the registered email confirmation which they won't have received if they were still pending.