This is obviously a cheap chinese product, and therefore there are low expectations of quality in the result. The same is still true but in a less obvious way even from more reputable channels like buying a USB charger from a shop in an airport there are so many ways that the implementation can be subtly not what you want and not give you the right charging speed with ONE of your phones. You can't read reviews of every product and even if you could, they're not going exhaustively test every last technical aspect of something.
My solution to this particular issue is that I buy all of my USB chargers from Anker (direct, not via Amazon). I know I'm getting a product from a company that has thought about it and tested it thoroughly, so my chances are much better. And the same is true with other products.