The preference you show in your screenshot is about replying (“posting in a topic”, not “posting a topic”), but the reason shown next to “watching” is that you have created the topic.
I don’t think there is a setting for the default notification level of topics you create. It makes sense to expect that, in general, users want to receive notifications about replies to a topic they create.
I just created an account at try.discourse.org, adjusted my preferences, and replied to a topic.
As expected, the notification level is “normal” after I replied.