Because Outlook has a feature, enabled by default, to automatically accept meeting requests with a status of tentative.

Signature
Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
http://www.turtleflock.com/olconfig/index.htm
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers
http://www.outlookcode.com/jumpstart.aspx
>I have observed my Outlook calendaring system place an event on a calendar
> prior to the event actually being accepted.
[quoted text clipped - 5 lines]
>
> Dave French