An MSF study found problems with specificity in eight rapid HIV tests. Additionally, gender and geographical location were correlated with false-positives. These are among the reasons for using confirmatory tests.
To repeat a line forever brought up by dissidents, there is no such thing as false-positives if HIV has not been purely isolated – as ‘discoverer’ Luc Montagnier admitted.
Simply, some of these tests are more capable than others of picking up probable biomarkers of health stress.