I think you mean the ratio of false positives to true positives (or 50% of the positives are false positives) for typical patients. The tests are generally extremely reliable, but the conditions are rare, so getting a false positive one time in ten thousand can yield as many false positives as true ones if, for instance, the syndrome has a frequency of one in ten thousand.

Note also that there are some only moderately reliable tests that are used only to indicate more invasive test. (For instance, triple test to indicate whether the fetus might have trisomy 21, followed by amniocentesis or CVS to check with high reliability.)