Good Receipt and Invoice Receipt Differences`

Dear Friends,

In Purchase Order History, I am seeing the difference like this

Good Receipt is 15,241,700 EUR under heading Quantity in OPUN.

Invoice Receipt is 18,138,.710 EUR under heading Quantity in OPUN..

How to find out where is the difference amount?

Please mentioned any table or report, which i can mention to the user so that he is satisfied,

All the help will be greatly appreciated.

Regards

Sridhar