Exchange Rate on inventorization different than PO receiving. This leads to a balance in Receiving I

While doing receipt against a foreign currency PO, the exchange rate used by system on Receiving Inspection stage is different than Inventorization stage.

The Exchange Rate type is corporate

Question: Why, if at all, does system allow the exchange rate on receiving to be different than inventorization in any case?

Example:

PO Rate = USD 5  / unit

Qty = 1

Ex Rate: 1 USD = 100 PKR (functional currency)

Receiving Accounting Entries:

Receiving Inspection          500     Dr

Accrual                                                       500     Cr

Inventorization Accounting Entries:

Inventory Valuation               510     Dr

Receiving Inspection                                   510     Cr