PPV Adjustment on Bill

I am currently encountering a specific case regarding the Purchase Price Variance (PPV) logic on the system and would appreciate your insights on it.

Here is the exact sequence of documents:

1. Purchase Order: Unit Cost = Y

2. Purchase Receipt 1: Unit Cost = X

3. Purchase Receipt 2: Unit Cost = Y

4. AP Bill: Unit Cost = Y (applied to Purchase Receipt 1)

When I Save the Bill, Estimated PPV Amount = 0, I Release the Bill, the system automatically generates and recognizes a PPV Adjustment.

Could you please help me understand under what specific scenarios or configurations the system behaves this way?