Question about test lines and levels

[greengranny]

Ok, dd got a very faint positive on a test Christmas morning. She then bought 3 First Response tests to do every other day to make sure levels were rising. They got slightly darker each of the first two times, but today's test was a little lighter than the one from two days ago.

Is there anything that could cause this other than dropping HCG? Could it be less concentrated in her urine? It was only after about 5 hours and she had had a lot of water before bed.

Would those things have any effect?