It's an excellent observation you made. Here's the deal, throughout the day, your weigh changes a lot. Your body is burning fat, you eat, excrete, grow, and you shrink while you sleep (a very small amount, but it does happen). So you're going to weigh different amounts at different times of day. The best thing you can do is weigh yourself at several different times throughout one day, add them all up, and divide by the number of times you weighed yourself. By doing this, you will get your average weight. What most people do is weigh themselves, and let's say the scale says 127, well then that person would just say "I weigh around 125." Because fact is they'll weigh both more and less than 127 at various times of day.