Why do Acquisition Date / Placement Dates change after asset creation

A User created a zero-cost asset manually in AM and back-dated the Acquisition Date (Dec 12) and Placement Date (Dec 12) to reflect known information.  When an AP transaction was processed and the cost appended to the asset the Acquisition Date and Placement Date changed to reflect the Invoice Date (Dec 16) of the voucher and overrode the Date established during asset creation.

Why is the asset Acquisition Date and Placement Date changing after the asset was created?