by Juan Leyva.
The Mobile app uses the core_enrol_get_enrolled_users method for retrieving all the users in a course.
This method checks groups settings, so you should see only the users in groups you can see.
In the Web Interface when you see the participants list you can see them filtered by group, so maybe that's what happening in your instance (you see only the users in a group but you can select other gorups), in the app all the users are listed (without the filter possibility)
Regards, Juan