Why would fat percentage change a.m. to p.m.?

I use a scale that measures both weight and fat percentage. I used to weigh in the evening, but now do so in the morning. We all know that you tend to weigh a little less in the morning, but the fat percentage readings are also (vastly) different in the morning than in the evening. In the morning, the fat percentage reads about 40%, but in the evening it reads around 29%. Does anyone know why there would be such a wild swing in the space of 12 hours?

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