Can it be because I sign up via Apple ID and he made a chess.con account directly on the Plattform?
It occurred because you gave different answers to the question about your chess strength. I think it's a multiple choice- new, beginner, intermediate, advanced, expert. The initial rating is assigned based on the answer.
There's no need to worry. Ratings change quickly when you are new. If either of you is mis-rated, the rating will soon adjust to the appropriate level.
Can it be because I sign up via Apple ID and he made a chess.con account directly on the Plattform?