Dear @Marten_Pfannenschmidt - first of all thank you for your reply
if you look at the first image (the scorer node accuracy statistics) you will see that I reduced my dependent variable to 1 and 0.
0 is for actual show-ups and 1 is for no-shows - if you look at each of these and sum up their TP/FP/TN/FN you will get 488 which would be the exact number that I used for my test-data (20 % of 2440)
the doctor’s question is why it works like this, which I can’t seem to explain (sometimes in this field you figure out that you have a missing understanding of something that is vital to what you’re doing which is very humbling)