I'm not sure that it works in quite that way, I think the double \\ is more likely telling regread and regwrite not to perform its substitution for the current SQL version and instance name.

The simplicity of these extended stored procedures is that you can use the generic key root and let SQL work out exactly where the registry key is located, then your code becomes portable.

So

“Software\Microsoft\MSSQLServer\MSSQLServer”

becomes (in my 2008 R2 default instance)

“Software\Microsoft\Microsoft SQL Server\MSSQL10_50.MSSQLSERVER\MSSQLServer”