Hi
Check this customizing point: V_TKEVA03.
If you set in your rules read the Standard Cost, system look for the Standard Cost Calculation according to the customizing (V_TKEVA03) not the field in the material master (mbew-STPRS)
A typical reason to work with Standard Cost Calculation, is bec you want to transfer the CostComponent Split to PA.
Wish this help you.
Arturo.