How are the conversion overhead rates on PIDs determined in a Standard Cost environment

We are seeing discrepancies between the Standard Cost Conversion Rates and the rates used when the PID is processed.  Since Routings do not have effective dates or Cost Versions, what determines the rates used by the PIDs?  What would cause the Standard Cost conversion costs to be different from the PID conversion costs.  We have not used the Manual Update Conversion Costs.  Do we need to run this to update the Routings on the PIDs?   If we rollup costs but do not run the revalue process, how does this effect Production PIDs?