Hello EMG:
It seems that your SubVI does not save the previous values like your "root" VI.
Whenever your "root" vi loads the SubVI, it sets all of your controls to its default value. In this case, it sets the variables "Mittelwert" and "Mittelwert vorher" to their default values which is 0.
So whenever your SubVI is loaded, the initial values of "Mittelwert" and "Mittelwert vorher" is zero, and when the "Clear/Init" is set to false, there will be no action, thus the values of "Mittelwert" and "Mittelwert vorher" will not be changed.
As a solution, I suggest that you would save the values of these variables in a shift register; similar to what you did with "Content". For better understanding, please see my attachment.
I hope that this helps.
Good Luck