svn+ssh configuration

I have Coderviewer server that is running on Local System account with desktop interaction. There is set environment variable SVN_SSH with "C:\\Program Files\\TortoiseSVN\\bin\\TortoisePlink.exe" value. There is also Subversion config under C:\Windows\System32\config\systemprofile\AppData\Roaming\Subversion path that contains ssh entry with ssh user and password specified under [tunnels] section and when it's specified like this it still ask about user name and password

ssh = $SVN_SSH -l user_name -pw user_password

but when I get rid of environment variable and replace it with it's value it works like a charm

ssh = "C:\\Program Files\\TortoiseSVN\\bin\\TortoisePlink.exe" -l user_name -pw user_password

I know it's not a big deal but I'm just curious why it's not working. Is it something with repleacing environment variable in ssh config entry?