Unable to connect to SQL server 2008 R2 via Instance name? I don't understand : 

I've installed sql server 2008 R2. the installation process asked me for the desired instance name  , so I wrote: MSSQLSERVER. 

after the installation , running Setup Discovery Report does shows me the instance name : 



I've also installed all the service packs.

Also , all the services are up : 



Also , all the ports are open ( tcp:1433 + udp :1434)





Also , Named pipes are on : 

So ,  where is the problem ? 

I'm unable  to connect via the instance name :


Also unable   with : 

user\MSSQLSERVER where user is the computer name.

Also unable  with : .\MSSQLSERVER

But (weird) : 

Those 3 do work : 


user ( "user" is my machine name)
(local)
.


Question : 

why I can't use my instance name to connect to my sql server ?

Related info : 

After connecting via one of the 3 who work  , I run a helpful info :
You selected the default instance name as the name of your instance.

Instance Configuration

"If you specify MSSQLServer for the instance name, a default instance will be created. For SQL Server Express, if you specify SQLExpress for the instance name, a default instance will be created."

To connect to the default instance you don't specify the instance name, only the server name.