Conductivity, which is the primary thing measured, changes with hydration level and will affect the readout.
Use it for trending, trying to keep circumstances before measuring as like as possible, rather than looking at the absolute value.

Went through a big weight loss/diet/exercise regime 11 years ago and graphed the various values the scale spat out. %Fat was a quite jagged one depending on how much I'd been drinking etc.
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/Michael