Why do some tests show evap lines but not others?

I’ve tested in the past with the same brand of test and an hour later the negative was still negative. This time a positive line appeared after sitting for about 30 minutes. I know that invalidates the test but just wondering why it did it this time but not all the time? I still have 11 days until my period so I wasn’t expecting a positive just showing someone that I wasn’t pregnant and now I’m nervous lol. and I’m almost 100% this test says negative. This is only the before photo and it looks negative to me. Just curious as to why a second line appeared and looking for advice/similar stories.