Windows Mobile ActiveSync:

When Windows Mobile ActiveSync device is registered, it retrieves the items from the server, and sends the items that were on the device before registration, to the server.

The items are removed from the device. 

Workaround:  To get the items which were sent to the server back on the device, remove the account and reregister the account.  The items, which now reside on the server, will synchronize to the device. Calendar items must fall within the synchronization range, however, in order to synchronize.