Hydration levels can cause it to fluctuate dramatically. Most are also calibrated for an "average" level of fitness, and state that if you are an athlete, the n

Hydration levels can cause it to fluctuate dramatically. Most are also calibrated for an "average" level of fitness, and state that if you are an athlete, the numbers may be inaccurate. So if he went from typical fitness to "athlete" fitness, or typical hydration to shortly-after-long-hike hydration, the error range for the second may be much wider than the first. That said, he undoubtedly lost fat. The BF% measurement change is likely not an accurate measurement of how much he lost.