Why are there different costing layers on a return to supplier against a PO when we use actual cost?

Summary:

For item 11121:

We brought in conversion 7 at 15.19 each.

There was a movement request for 4 that depleted inventory 4 @ 15.19

Then there was a receipt for 20 on PO CNO10118976 @ 63.54.  There was a return to the supplier against the PO for 18. 

In this case it used cost from the 3 old on hand and 15 from the PO.

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