Ah, from this comment:

Sunscreen works, just not if you buy it from shady manufacturers that try to maximize their profits and care about nothing else.

I thought you were saying "don't buy knock off brands and you're safe. When actually you're saying everyone is cutting corners.

Unfortunately the independent testing here happens infrequently (no more than once a year), and it's different brands failing each time.

In general, the failing brands are testing as much lower than their stated SPF ratings. As a consumer, the best chance is probably to buy the highest rating you can find so even if it's lower than stated it's still pretty good.