COGS G/L entry

A Cost of Goods entry posted in MARCH 2013…and took COGS Report…when we run a report again now in JULY for the same entry it shows a different COGS value…what is the possible chances to happen like this…

which costing system are you having ? In case of LIFO COGS might not be the same.

Average Cost…

Is there any return happen against that invoice ???