Vvfl??

When people post photos of a very faint, almost non existant, line are they testing before or after missed period? I took a test a few days back and I am pretty sure it was negative. Granted I wasn't hoping to get a negative, but maybe it was an extremely faint line that couldn't be seen unless I really wanted it?? I was testing when my period was due. Just curious why some people get faint lines and other are obvious?? Thanks!