Since you are measuring laying down, there is an angle perspective during measurement. If you do the same for the water, you see refraction index that might make you look longer and larger. When you lay down, the blood pressure pushes your penile shaft a bit more to make it appears to be larger. When you stand up, pressure is diverted to the main organs to ensure equalization of pressure through capillary effects, mostly bypassing the penis.
If you get an erection while standing up, only a portion of the dedicated blood volume will be trapped in your penis through veinous constriction. When you lay down, the system doesn't worry about pressure differential and send as much blood into your penis as you want and can hold. This is why 69 position make your penis looks more enhanced.