Lines getting more faint

Ladies my top dark test is from weeks ago. I’m 8 + 3 weeks today and decided to take another test yesterday and today for fun. My lines look lighter? Why would they look lighter than 3 weeks ago? I’ve already had an ultrasound and the babies heart rate was 153, so everything looks good. Just wondering why my prego test would go lighter? I would think it would be a dye stealer right now this far along. Thanks