I'm confused why a very faint line?

Since joining this app I see most people on here are TTC and they post these pregnancy tests asking if it's a faint line and most of the time I don't see anything period and feel like they're faking themselves out...but that made me wonder. 🤔 I took a test the day of my period was to start (see below) and they were as clear as day so I thought all tests were like that. Why is it some people can be late for their period by a week or so and the line be so faint? Just curious.