Showing wrong amount after partial receipt of PO

we create a PO for an  item A, having Qty ordered 1680, unit cost 13 rs ,Extended cost 13*1680=21840 Rs

Now we do the partial receipt of Qty 210, So that the Open Amount should be 210*13=2730 and received amount should be 2730.

but in F43121 it showing  21840 as Open amount and Received amount.

When we again do partial PO receipt of Qty 70 So that the Open Amount should be 70*13=910  and received amount should be 910.

but in F43121 it showing  19110 as Open amount and Received amount.

When we again do partial PO receipt of Qty 70 So that the Open Amount should be 70*13=910  and received amount should be 910.