Matching Rule RULE_P400 does not seem to be triggered by the AP_MATCH process

Please refer to the below steps:

1. Create a PO with Quantity 10 and Unit Price 25...So the PO Extended Price is 250...
2. Fully Receive the PO
3. Create a Voucher by copying the PO
4. Change the Unit Price to 30...So the Invoice Price is 300...
5. Run the Matching process
6. Match Exceptions are for RULE_E200 and RULE_V400...Both are related to Total Price on the PO and Voucher

The question is why the Match Exception is not there for the RULE_P400 which is related to the Unit Price?

Thanks

Nadeem