Difference between Local Admin and User part of group Admin ?

Hello Friends,

I am facing a unique problem. My Installer is suppose to work with administrator user.
The scenario here is, when user is logged as local administrator, all goes well..
BUT, when logged in user is part of Administrator group, and invokes the setup, it fails in performing some actions due to insufficient admin privileges..
Why is it so? I am not doing anything specifically for this behavior,I have just set the requires administrator privileges settings in Release section.

My question is - Ideally both Local Admin user AND user part of Admin group, should have equal rights, but then why the behavior is different in Installshield?

Any thoughts please.

Thanks