The “20% extra free” marketing ply has been around forever. I reckon that the cavemen used to encounter it when they went foraging in their local area. “Pick the fruit from this bush and get 20% extra berries free!” Or maybe not, but you get my point. It’s not a bad tactic: everyone likes to get more for their money.
Only when that’s actually the case, though, otherwise you’re liable to feel duped, annoyed, and never buy for that company again. I’ll admit that math is not my best subject, but even with my rudimentary skills, I realize that the same size can can’t hold both 30% and 40% more soup.