Unit cost on Value per Period not equal to cost set 1

I have a question around Standard Cost and Inventory value per period.  We are using standard cost for our valuation method, and I would expect that the inventory value per period unit cost would equal cost set 1 for all parts, but I am seeing several parts that are not equal.  I have started to dig into the issue, but I have not had much luck understanding why the unit cost in inventory value per period keeps changing each month.  Any ideas?   I am thinking maybe it is a part or company setting that might have be reviewed.  Thanks in advance for your support.   Bob