Answer: 

The behaviour for Last Cost calculations in product records is that the Last Cost field will replicate the product's Average Landed Cost (ALC) if the last Purchase Order (PO) that was last completed included a re-costing.

i.e If the products were confirmed receipted and then the applicable costs were completed at a later date for the same PO.

The reason for this is to ensure that the Last Cost that's advised is as accurate as possible within the system at all times, otherwise, the Last Cost would only advise the unit value of the re-costing amount.