Too be fair I doubt Amazon is a good judge, they often run out of stock of this or that SKU which may affect the ranking.
I would think keeping a sku with a physical disc in stock would be harder than a digital insert sku?! As far as I can remember (could be wrong), the Uncharted Collection Bundle has lead the digital sku month after month on Amazon. Actually at one point in time (November I believe) it was sold-out, and the digital sku was only available.