Their survey wasn’t perfect, namely because they went book shopping in July. Shopping for books in July doesn’t give you perfect insight into what prices are like during the beginning-of-the-semester rush, but summer classes are a thing, and the sites’ inventory and prices did vary between visits.
What they found was that the site that had the consistently best prices was Amazon. At any given time, a different site might have a slightly lower price, but Amazon’s prices were the best consistently over time. Barnes & Noble was consistently the highest, which isn’t surprising given that the company owns a huge number of college bookstores.
New books were trickier: there wasn’t much variation in price, but the very freshest texts weren’t always in stock at all sites.
The overall lesson from this experiment: give yourself some time to shop for books and find the best deals, and also be sure to shop around for the best prices.