Though serial costing is enabled, Sometimes Serialized items use the Average Cost

Summary:

Though Serial Costing is enabled (Setup / Inventory / Control / Main tab), Sometimes Serialized items will still use the Average Cost.

Cause:

By design.  

Solution:

Serial Costing is supported only for Always serialized items. 

• Always Serialized items use the Serial Cost
• Sometimes Serialized items use the Avg. Cost