Do you have price per unit on the price tags in your grocery stores? They have to show that by law in my country, not sure if it makes a huge difference because not everyone knows to compare though.
I don't know whether that's done intentionally. Hanlon's Razor says to assume not without proof.
A pet peeve of mine is tissues/toilet paper/paper towels. Sometimes the price is "per roll", sometimes it is "per sheets". Sometimes it's even different between different package sizes of the same product. It's infuriating to have to bust out the calculator to figure out if the deal on the 6 pack is a better price than the regular priced 12 pack.