So your accounting entry at the time MIGO - Goods receipt will be
Inventory account Debit (material price X PO quantity) 3.69 X 100 = 369
GR/IR account Credit (PO quantity X PO price) 3.5 X 100 = 350
Price difference Credit (Material price - PO price X PO quantity ) (3.69 - 3.5) X 100 = 19.
And it is happening because of you material price got differ with PO price and your material is having the standard price. If you material has the standard price, then system can not able to change the standard price.