Unable to connect to SQL server 2008 R2 via Instance name?

Question

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 :