Office Outlook 2003 sp3

We have about a hand full of computers where an instance of outlook remains active after closing outlook when an email has been opened up.

In most computers the outlook application closing will close the email screen as well and will remove the instance of Outlook from the task manager. However, we have about 5-7 computers where an the instance will remain. Then when outlook is opened again, we have 2 instances in the task manager. Continue the same thing we can get multiple instances remaining. (we have had users with over 20)

Any idea why some computers do this while others do not?

Thanks in advance