@Nicolas_Swisscom,
Having a specific "set control" vs a "random control" changes the way the results are displayed. The OOTB setting is to use random control however the KB you have referenced shows what it would look like if you were to have selected a specific experience as the control. I can see how this is a bit confusing at first if you weren't aware of the difference.
Please let me know if you would like me to clarify anything. If you reply please tag me with @MihneaD so that I see it.
Mihnea Docea | Technical Support Engineer | Adobe