One warehouse or is that same lot in stock in different warehouses?  Is SO Unit of Measure the same as PO Unit of Measure or are they different?  Are you sure the item master is flagged as LOT Valuation method?  What does the Cost Tier Screen look like?  

A few more screenshots. Yes, it is using Lot valuation. Same Unit of Measure for SO and PO. I also checked SQL server to verify.

What you are showing should be impossible. Any enhancements? 

The current "value" in the Lot should be shown in IM_ItemCost.  Is that value correct?  How does that compare to what is in the IM_ItemWarehouse file?  You might also want to review the results of *UTL (from File - Run...), inventory rebalancing utility.

UoM problem is a good theory.  Check the UnitOfMeasureConvFactor in AR_InvoiceHistoryDetail and PO_ReceiptHistoryDetail.

Check the data in IM_TransactionHistory, compared with IM_LotSerialTransactionHistory. 

Look for bad data in IM_DataEntryCostCalcCommit... this is the table that locks to inventory during the posting process, and sometimes has orphaned records if an update crashes.

In the tables that have "TierType", check the codes in the File Layouts to ensure they are correct for Lot valuation.  Bad TierType values can cause weird things.