Faint lines for all pregnancy tests?

Joanna • I am 22 years old, nursing student, and so in love with life and it`s twisted ways. Peace, love, and light guys! ✨
So I read a comment where a woman stated that in all her pregnancies the necessary second line on the test would always be faint.... 
My question is, why does that happen?
*And also, has this happened to anyone else? Where you're pregnant and the pregnancy tests come out with a faint line..