Because the lines are on the indicator strip. One is a control, the other is the result. If you saturate the strip, you will see the result line bleed through. You can do the same thing with a pregnancy test. Dunk it under water and it will show the positive indicator just due to being saturated.

Kind of like a wet t-shirt contest. The nips are already there, but it isn't until you wet the shirt that you can actually see them.

Just to clarify, it isn't that he got it wet. It's that he got it saturated.