This is far from the first price comparison to make such a predictable discovery, according to a report by Time. For example, Cheapism found that 14 out of 20 store-brand items were cheaper at Trader Joe's. Trader Joe's apparently trades big stores full of lots of brands for smaller, barebones outposts with fewer offerings in order to undercut its competitors' prices, according to the report.
The SFGate writer also points out that you can do significantly more damage at Whole Foods thanks to its ultra-pricey high-end products: "Bottom line: At Trader Joe's everything is affordable while Whole Foods has some fair prices if you stick with the basics, but your wallet will shrivel if you want to put Himalayan yellow raisins on your oatmeal (I bought these once without noticing the price and later looked at my receipt to realize they cost $16. Ouch!)"