well frer is not a quantitative diagnostic tool, just a mere colorimetric test strip, and despite being a really reliable one, i often find it too "over-considered" and overrated on this forum as some kind of a gold standard. that test is very popular in UK&USA but barely to be found in other markets in EU and i think people take it a bit too seriously sometimes.
there are MANY things that can influence the test result and honestly i've never seen a frer where the control line is lighter than the test line.
starting just from merely your urine being more diluted than usual, to lower temperature in the room while you're testing (the temperature is actually one of the KEY factors in these reactions and if your bathroom is cold or the surface you placed your test on is, then you might get a fainter line), to stuff like hook effect or just a plain over-saturation of the test.
also, your body also produces several different types (isoforms) of HCG and the test manages to detect only one of those; the proportions of different isoforms greatly vary within the first weeks and this can also contribute to what you've seen.
i really really wouldn't stress over it. it is just a stick you pee on.
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