This comes up about once a month it seems.

The Tantia scale is not a highly accurate device. It is best used to monitor trends over time.

The scale (at least mine) is very sensitive to hydration level. It is best to use it in the afternoon or evening at a time when you can have a relatively constant hydration level. Using it after exercise is not a good idea! In the morning it will likely read a higher BF% than later in the day because you are dehydrated when you wake up.

Perhaps newer Tanita scales are better about this. Try using it over the course of the day and see. Regardless, don't freak out over day-to-day changes.