I have read some think those curved FRERs don't actually have an hCG threshold, so can detect even lower than 6.5miu. I don't know how true that is of course, but it's a theory and would answer why so many see lines which don't become anything.
However, I had a similar thing this cycle. My first IC I was about 7dpo, it was clear. Pink. The right width. Then I didn't see anything for a few days, followed by some obvious positive tests but then blank tests of another brand... to save the headache, perhaps it's best to wait a few more days and then test. Even on a curved FRER you should have a reliable line there if you are pregnant. Hopefully things are fine, sometimes you won't see a line due to it being so early and diluted urine and stuff