I tried the experiment again. Weighed self before bed. When I got up, I weighed myself again, (before urination). No change. Weighed after urination. No change. (Scale measures down to half pound.)
So, huh?
There is an adage amongst biologists: "Given tightly controlled conditions of medium, humidity, light, temperature, and air pressure, the organism will do whatever the hell it wants."
Did you make sure you ate the same foods and drank the same liquids at the same times of day in both trials? Was the room temperature the same? The relative humidity? If you didn't control the environment, it may have impacted the results.
Even if you didn't control the environment, your body is a biological organism - it is dynamic, not static. You were not likely to be in the same state the second try as the first, and so maybe you held on to your water better.
We see here why science and statistics go hand in hand. One experiment that happens to go as per the hypothesis is
not proof the hypothesis is correct. It may be suggestive, even demonstrative, but it is not proof. Why? Because the real world is not theory. Real world data never matches theoretical predictions to infinite levels of precision. There is variance and scatter. Scientists do experiments over and over to help average out the random elements.
When someone says, "People lose two pounds of water over night," that is probably not a 100% surety. At best it is on some sort of broad average, for some assumed normal conditions. Sometimes it'll be less, sometimes it'll be more. Do the test 100 times, and you can come up with an average for what you lose. Do the test 100 times each for 100 randomly chosen individuals, and you may start to get an average for what people do, in general.