I've wondered this..

You know how when people are asking about ovulation using OPKS or wanting to shown progression of a darkening line day by day on a pregnancy test or OPK, they line up all their tests that they've kept, label them with the day they were taken and take a picture? I thought tests weren't supposed to be read after a certain amount of time? Doesn't the line fade or the test become invalid? If so, why keep a weeks' worth of tests to line up to compare? It'd seem to me that the only one that would be valid is the one you just took. Or am I thinking about this wrong?