How does group membership work?

If I have a parent group say "All Users", a child group "Admins" and an object say a rule "A" which has following security settings on it:

User or Group
Permission Level

Default (All Other Users)

No Access

Administrator

Viewer

Now, as per top to bottom inheritance approach, parent group "All Users" has "Viewer" rights set for rule "A" and it's child or member group "Admins" should also get "Viewer" rights but as given explicitly here "Administrator" rights to "Admins", it overrides the security setting "Viewer" passed to it by parent "All Users" to "Administrator". 

Shouldn't "Admins" also get "Viewer" rights to the rule "A", how it gets "Administrator"?

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