Increase In GR value for the Line Item

Dear Friends

I have a Cost center PO with "text item" ordered for Quantity 10000 PC for an amount of 10000 USD. MIGO has been done 4/5 times where the quantity inwarded equalled the GR value. in the month of Novembeer the user carried out GR for a quantity of 290 PC but suddenly the GR value was higher i.e 434 USD. User cancelled the document using MBST and the Value which got cancelled is 290 usd only and not 434(which should have been).

My query is why system behaved abnormally? There is no item change log in the PO as User has not changed the unit price for the PO. I have checked the PO history Table " EKBE" and i do not find any changes to the Value of PO during or before the period , no changes are displayed in the Table w.r.t value.

Secondly why during cancellation system only considered the PO value not the GR value. Where should I look for the reasons of this abnormality.

Thanks