Don't worry. I have asked myself the same questions. The system used (BMI) has many variables that affect the measurement and one of them is the water present in the body. In summary, the differences in measurement are normal and in no way do you have to forget that it is not a professional device. I recommend taking measurements in the morning, shortly after getting up and weekly, and not after a shower or physical activity.

For better results, you need to buy a scale that not only measures the feet but also the whole body by means of handles. Papiccg has already answered your questions about these scales.