Strange Pricing

Seattle WA
Was at Business Costco yesterday and of course cruised the oil aisle. They carry both Pennzoil and Chevron oils in cases offering several different choices in viscosity. In the past, they've always been the same price but yesterday I noticed the 10w-40 weights were a dollar more per case. Never saw that before... I suppose it's because the lower sales volume of those may be driving the slightly higher price? Or does a 40 weight oil cost more to produce? Donno...