How important is FMU?

I had taken a FRER a 2 days ago and it was FMU. The test did actually have a vvfl but I somewhat believe it may have been a fluke. I took another FRER today but I didn’t use FMU(I had to pee way too bad) and the line came back even more faint than the one before. If the test from a couple days ago was actually positive, it would have been darker today even if it wasn’t FMU, correct?

I’m really starting to think all the FRER are doing this nonsense.