When you are going about the day, you eat food and drink liquids. The water and nutrients go into your body, especially the muscles. So, you'll tend to weigh more in the evening. If you weigh in the morning, you haven't ate, your body has dehydrated some, so it's a "drier" weight.
I lift weights, so I drink a ton of water to keep my muscles hydrated. I can weigh up to four pounds heavier in the evening than in the morning.
Remember, water weighs 8 pounds a gallon.