With digital weighing machines, sometimes weight fluctuations take place because the main screen number is not 0.0 before weighing. It may already be a negative number like -0.7 or may be a positive number. This adds or subtracts from our actual weight.
I once checked my weight and it showed 3 kg less in just 2 days. I just couldn’t believe it but then i fixed it to a 0.0 and it showed the correct result! 