Best Is Average, Better Is Best
According to US government regulations, certain products are considered commodity items (in technical jargon, parity products), with no noticeable variation in quality from manufacturer to manufacturer. As far as the government is concerned, there is no functional difference between one product and another in a given parity class. Aspirin, toothpaste, toilet paper and a host of other consumer goods fall into this category. Economics would call this item one that causes Perfect Competition, so manufacturers, to sell their brand, must differentiate it from the others.
Paradoxically, this allows a manufacturer of any of these items to legitimately and legally claim in advertisement that their product is the best. The logic behind this is that if all varieties of aspirin on the market are equally good, then they are all the best by definition.
The really counterintuitive part is that the makers of these products are not allowed to claim that theirs is "better" without substantial proof that it is somehow an improvement upon the commodity level.
The result is, in American advertising at least, that "better = best" and "best = mediocre".
In German television, this problem is often solved by claiming a product of some category is better than "ordinary" products from the same category, which is basically a way of saying "better than crud".
A similar phenomenon is the extinction of moderate adjectives. No longer are drinks available in small, medium and large; now you have a choice of large, huge, gargantuan and colossal!
Compare Absolute Comparative, when "better" is liberally spread without saying what it's supposed to be better than.