Ok, I have installed the Membership plugin and maybe I am missing something.
I configured the plugin through the Wizard, creating a Member subscription level.
It doesn't seem to me that the plugin creates a WordPress role, at least not in my installation. As a matter of fact, I don't see the role Member listed anywhere in the WordPress installation.
From my understanding, users can be Members (of the Membership plugin) regardless of their WordPress role. As a matter of fact, after installing the Membership plugins, all my users got activated as Members (either administrators, subscribers and editors, which are the roles that I am currently using).
Therefore, it seems that the two attributions - WordPress roles and Membership levels - are completely separated.
Now, I am trying to understand why you see a role "Member" in the list of WordPress roles. Is it possible that you created that role yourself using another plugin while trying to configure your Membership plugin?
Or can you try to let me better understand how the Membership plugin interacts with the WordPress roles. From what I can see in my installation, the two concepts (WP roles nad Member levels) are completely independent.
This is a test you can try to better understand the situation.
Go to the Users page (/wp-admin/users.php): what do you see under the column "ROLE" for the users that are definitely Members according to the Membership plugin?
If you still see standard WP roles, than it is pretty obvious that the Membership plugin add something completely unrelated to the WordPress roles.