This is the way it works. When a user has a role at the system level, such as Manager or Course creator, and are also enrolled in a course in a course level role, then the Participants list in the course shows both the system level roles and the course level roles.
This is useful in case you have a system level role and a course level role that may conflict on certain permissions and you need to troubleshoot the issue. For example, you may have a system level role that allows someone to see but not edit grades and this is not working properly when they are also a teacher in the course. You want to be able to see what roles are "above" the course at the system level since those permissions are inherited.
In the case of the standard Course creator role, this is a system level / category level role only - by defintion. It would make no sense to have a Course creator role at the Course level since you have to be outside of the course to create a new course. There is no conflict of permissions in this case between Course creator and Teacher. Everything is working fine, as you say.
So, the teacher is not "marked as course creator for that course" -- what you are seeing is the system level role being inherited and shown in the Participants list.
I hope this is clear.