I have done some further research and discovered, that I made a mistake. My calculated sensor got it’s value from an energy sensor. As I did some further research concerning utility meters and energy sensors I discovered, that energy sensors often do not constantly increase its values but in-/decrease in very small steps. Analyzing my sensor, it showed exactly this issue
These small in-/decreases result in the difference between an utility meter with an w/o net_consumption. See also [Utility Meter] Daily sensors are not calculated correctly