HIV testing | what makes index value vary

I tested for HIV at 5 and 9 weeks results were non reactive 0.24 and 0.23 respectively, but then I tested again at 12 weeks at a different place and it was 0.62 non reactive. Can someone please explain what this number means and why would it change between tests? since then I have done one more rapid test 4th gen which came back negative at 16 weeks.