Most of us (atleast me) forget a simple stuff around SharePoint groups often ...
SharePoint Groups are defined / exist at the Site Collection level. If we need different group members in a subsite then we need to create a new group for that subsite.
Sharepoint Groups are created in SharePoint Site Collection context.
So when the users are added into SharePoint Groups will be reflected everywhere. so add specific subsite level groups after setting the Unique permissions.
For Eg: If you have 2 SharePoint groups (Admins, CommonUsers) at site collection level, when users added to these groups from anywhere (from top level site or sub siites) in the site collection will be reflected everywhere. You are advised to set unique permissions to subsite, then add sub site level groups (SubSite1 Admins, SubSite1 CommonUsers) and add users to those groups.
