The Culprit?

Well in this scenario, ActiveSync is not to be blamed.  The Archive feature in Outlook looks at the date/time of the message's Sent, message's Received or at the message's Modified date/time, whichever is later.  So although the date of an information type might fall under the date you have set, it will ignore it if it has under gone any changes on a date later then the one specified.  This would also include changes made to the information types via your Pocket PC.  ActiveSync updates the information in Outlook and therefore the modification date is altered.

For more related information, refer to the Microsoft Knowledge Base documents 295657 and 260217.