Yes there was a problem with dodgy counterfeit certificates in Dubai a couple of months ago. And I’m sure in many other places too. Also, testing around the world is done in different ways, none seem as thorough as Australia. From personal experience, some countries use one nostril jab only, others do the 2 nostril and throat but are way less vigorous (deep) than here, so false negatives are more probable. But the more likely reason is that, as SM says, all a test does is say you’re not positive at the time of the test. The incubation period is at least 10 days in some people may not show a positive between infection date and symptoms (if any)..