I believe the average days shipped is sum of days between each shipment divided by the number of shipments.

What I don't know but suspect is happening, are credit notes and adjustments included in this calculation or just invoices.

This number is dependent on Period end and Year end being run in IC on time every time.

If you have Toolkit for Adagio, on a copy of your data try running the "Recalculate Inventory Sales History". If that corrects the result then Period end and Year end have not been run on time every time.