They do an absolute shit job of it, because there are no standards or regulations. Most times I see this, similar items next to each other on the shelf are using different "standard" units making comparison impossible (e.g. price per ounce vs. price per piece). I've even seen two sizes of the same brand/product use different "standard" units on the grocery store price label.

It's as often design to mislead and prevent comparison as it is to inform you that "store brand is definitely cheaper than name brand".

Since none of it is mandated, they do only what benefits their bottom line or they are publicly pressured to.