Why is the MB51 standard currency amount different from the Purchase order history standard currency?

Translation

Why is the standard currency amount of the material voucher sometimes inconsistent with the standard currency amount of the purchase order history item?

Reply:

There are 2 fields corresponding to the FI storage position. The field of the item voucher (Mseg-dmber) is the amount of the inventory account that is held. Purchase order history (EKBE-DMBTR) is the amount of Gr/ir accounts displayed when receiving goods. There are many reasons why the Inventory account amount is inconsistent with the Gr/ir account amount. For example, when an item is using S for price control, the price of the purchase order is not equal to the standard price.

Because if you use S for price control, the receipt is the value of the standard price as the inbound amount, regardless of the purchase order price, which causes the MB51 standard currency Amount and the purchase order historical standard currency amount is different results.

Why is the MB51 standard currency amount different from the Purchase order history standard currency?