typically when you add a login in an instance it applies for the instance.
If it is not SQL 2008 -
If its a domain login that is an admin on the server. i.e windows admin, then 'builtin admin' login in SQL grants the user rights by virtue of being Win admin across all instances on the server.
If it is SQL Server 2008 this doesn't apply because Builtin admin login isn't there by default like in sql server 2005