I recently had a neg hiv ag/ab 4th gen rfl blood test. why does it say that a nonreactive result does not exclude it?

Lag time/window: If the test is done too soon after exposure, negative, non-reactive, results do not HIV as the body has not had a chance to react to the infection. If the test was done more than three months after an exposure, then you can be more or less sure that you are free of the infection.

Could be too early: Because the test doesn't become reliably positive until 4 weeks after exposure. Before that time, a nonreactive test result doesn't prove someone doesn't have HIV. But after 4 weeks, a negative 4th generation test is conclusive proof.