I'd have to read the study to answer you more intelligently. And I'm too busy pestering people with psych questions :)
But, in general, all tests have a different level of sensitivity. It may be that the normal test is cheap, easy, but not 100% correct, but the alternative gold standard test (if one exists) isn't generally used b/c of costs or time involved or other factors like that.
Like, the quick AIDS test is easy, cheap, and quick, but it does generate false positives. The gold standard test is much more sensitive, so if you do get a positive on the first test, they'll retest to make sure you didn't get a false positive. I can't imagine what that would feel like, waiting between the 2 tests, knowing you've already come up positive once but aren't sure if it's for real!