There is a difference between Redmine users and users found on the People page. One person can be found in both places. However, Redmine started with a basic users list, that was available only for administrators. People plugin allows all users to be visible globally.
When you visit a person's page in People, there is a link to this person's page as a Redmine user.
When you click on it, you will see the Redmine profile's page: