Okay, let's re-iterate...
The actual question (and an excellent one at that)
"What does 'run as admin' do that being a member of the administrators group does not?"
(Answer)1. It allows you to call on administrator rights while under a user session.
Note: The question is wrongly put; one is a command and the other is a group object to apply policies.
Open a command prompt and type runas /?
.
This will list all the switches the runas
command line can use.
As for the Administrators Group this is based on GPEDIT or SECPOL and whether or not a Domain administrator is present or not or a network is present or not.
Usually these things will apply restrictions on computers that the administrators group is not affected by.
The question should be
What does runas admin do that run as user does not?
OR
What does the Administrator group do that a customized user group can't?
You are mixing apples and oranges.