Thanks a lot for your answer.
What I had as definition:
S(1,1) = Reflected wave (V-) / incident wave (V+) on port 1 (while all other ports are matched to the norm impedanze (e.g. 50 Ohms) (From Pozar "microwave engineering").
In this case S(1,1) = 1 (not 1 dB, but magnitude 1) means all the power is reflected from port 1.
As you explained, it seems a different definition was used for datasheet values.
Greetings,
Tom