When exporting contacts using "Export PRIMARY contact fields" , the Group(s) column includes groups for which the contact was previously a member.
When choosing the particular fields to export, the column only includes current groups.
Verified on the demo site just now ( http://drupal.demo.civicrm.org/ )
Just so a search for "Adams, Purvi" and export PRIMARY fields and you will see the field contains two groups but the contact is only a member of one of them.
This also happens on my 2.0.4 installation.