It's not obvious what counts as a "faint line" -- is this too faint? Is it not really there? Am I misreading natural variations in the colour of the test strip?
It's not a matter of reading instructions -- even intelligent adults who have followed the directions 100% correctly can still struggle with knowing what counts.
Anyone who has ever taken a pregnancy test (or seen someone else take a pregnancy test), can tell you about the major amounts of stress possible attempting to read a "simple faint line" -- see https://www.reddit.com/r/TFABLinePorn/ for one example.
I'm not saying that every test strip needs $20 of Bluetooth gizmos and e-waste. Just that normal, intelligent, fully-instruction-following adults can reasonably be expected to not always know how to read "any faint lines" accurately.