Physical inventory posting differences

I created bin PI.

Bin stock is as below as per system:

HU1: Mat=ZTEST1, qty: 10 Kg

HU2: Mat=ZTEST1, qty: 20 Kg

and

Loose material ZTEST1 of qty: 5 Kg.

Counter has counted HU2 =20 Kg and Loose material :5 Kg.

So the difference is: 10 Kg.

But while posting difference system calculates difference in bin as 100% considering book values of only HU1 instead total book values stock.

Can any one help.