A new product cannot have that thorough a testing...because if it did it would not be new but be used! QED.
A refurbished product needs more rigorous testing because Apple cannot trust a returned device the same degree it can trust a brand-new device straight out of manufacturing which never left Apple's manufacturing chain.
A brand-new product never left Apple's control, direct or indirect, so it doesn't need such testing in the first place, unless the defect rate is so high that there is a significant probability of getting two defective products in a row, which basically means either an underlying design or manufacturing issue is at play.