Purchase Posting System accounts difference is not 0

Hello Experts ,

Although the difference between Purchase Account , Cost of Goods Purchase Account , Purchase Balance Account & Purchase Return Account must be 0.

But at my client place i have a scenario where in the Difference is not 0 , this came in to light when we were doing the reconciliation.

I have a normal process here , Purchase , Goods Receipt PO , Landed cost and then sales.

But i am not able to figure out the root cause of such difference.

Any body can suggest or any body has faced such scenario , then please suggest what accounting logic works behind this.

Regards

Vamsi Krishna