Builtin\Administrators cannot login in SQL 2008 R2

I created a Windows Login, Admin2, and added that to the Windows Administrators Group.

I then added the Builtin\Administrators login to SQL and assigned it to the Sysadmin Server role.

I then logged into Windows as Admin2 and tried to connect to the instance. However I got the following error: 

  Cannot connect to myserver
  Login failed for user myserver\Admin2 (Error 18456)

In order for Admin2 to connect, I had to explicitly add it as as Login and assign it to the Sysadmin Server role.

I am on Windows Server 2008 R2, SQL Server Developer 2008 R2. Not on a Domain.

Any idea why Builtin\Administrators doesn't seem to work anymore?  Is it no longer associated with the Windows Administrators Group?

Thanks for any tips.