Quantity in Purchase Order Price Unit is different in GR

Dear All,

I am having an issue regarding to the quantity in Goods receipt and Purchase Order.

The quantity in Purchase Order Price Unit is different in GR. There is no any change for the quantity.

For example: The total quantity is 60, they have goods receipt for 3 times.

                    First GR: quantity is 20, quantity in POUn is 20

                    Second GR: quantity is 20, quantity in POUn is 20

                    Third GR: quantity is 20, quantity in POUn is 29

Would like to know why the quantity in POUn is 29 rather than 20 in third GR? Please advise.

Thank you.