If one product says "5g fat / serving [serving size: 40g]" and another says "4g fat / serving [serving size: 30g]", many many people will think that the second one has lower fat content.
If these were normalized to 100g, it would be clear that the first has 5/40*100 = 12.5g of fat whereas the second has 4/30*100 = 13.3g of fat.
Overall, humans are dumb and can't (or won't) do math in their heads. Consider the stupid thing with the failed 1/3 lb burger positioned in the market against the 1/4 lb burger ( https://www.snopes.com/news/2022/06/17/third-pound-burger-fr... ).
If one product says "5g fat / serving [serving size: 40g]" and another says "4g fat / serving [serving size: 30g]", many many people will think that the second one has lower fat content.
If these were normalized to 100g, it would be clear that the first has 5/40*100 = 12.5g of fat whereas the second has 4/30*100 = 13.3g of fat.