Can anyone explain this?

Pam

So I woke up at 4:30 this morning after going to bed at 9:30pm so it was an 7 hour hold. Then I woke up again at 8:30 and tested again. The top two are the 7 hour hold, bottom two are the 4 hour hold. Can anyone explain why the 4 hour hold is so much darker? (Have to click on the picture to see the top two)