The problem is that some grocery stores (or products, it could be either) use different quantities. For example, one product might be price per quart and another is price per gallon. I've even seen volume and weight being used for similar products.
I frequently shop by looking at the price per amount comparisons and it drives me crazy when you're looking at two different toothpastes and can't make an effective comparison.
It is my understanding that this meets the legal requirement, but also satisfies the business desire to confuse the customers trying to make a decision based on the greatest value for money.
