Issue: During installation of the Application Virtualization 4.5 Management Server, if you provide a server name\instance name for the SQL Server with a port number that is only valid for the Default Instance, then the Installer will connect to the Default Instance.  This will happen even if the Default Instance was not specified.

The reverse is also true in this scenario: If you try to connect to the Default Instance, but provide the port of a Named Instance, the installer will connect to the Named Instance database.

Cause: This functionality is by design. The problem only occurs when ServerName\InstanceName is entered in the "Host name" field as this field was never intended to support an instance name. 

Resolution:  To connect to an instance, you should provide the HostName of the database server and the TCP port number on which the instance is listening.  The InstanceName should not be provided as part of the HostName.

Source:→ App-V Blog