G/L picking during account assignment P & Q

Hi Experts,

Please help me to clear one doubt:

With Q system is picking G/L account as per OBYC settings corresponding to valuation class of material irrespective of what G/L I enter manually here, a default G/L is picked and its understood. But with P (same material code) system is allowing me to enter G/L manually and saving of that PR/PO.

Kindly suggest why system is not validating material valuation class -> G/L combination with "P".

Thanks & Regards

Saurabh