Sql Server Agent is part of the sysadmin role even tough Sql Server says it's not
After some reading I think I got it.

"NT SERVICE\SQLSERVERAGENT" is a member of the local group "SQLServerSQLAgentUser$RINUS$MSSQLSERVER".
This group has al the right that a sql server agent account needs

"NT SERVICE\SQLSERVERAGENT" is a part of the sql server fixed sysadmin role

"NT SERVICE\SQLSERVERAGENT" can't be selected in the list of available built-in accounts, local accounts or domain accounts. This is because they are services, not accounts. They have a security identifier (SID) in Windows, but Windows knows they aren't real users. Windows can authenticate them, but they don't have passwords that any human can use. If you run lusrmgr.msc and look at the groups, you will see groups like SQLServerMSSQLUser$computername$MSSQLSERVER and NT SERVICE\MSSQLSERVER is a member of the group.

The dedicated account that I have created for the agent is set for the windows service

If you connect the dots you will get a chain that explains this. The part thats new for me is that the NT SERVICE\SQLSERVERAGENT can represent the dedicated agent account.

Sources :


http://blogs.msdn.com/b/varun_sharma/archive/2007/08/30/how-to-run-sql-server-agent-and-sql-server-jobs-with-least-privilege-in-sql-server-2005.aspx
https://social.msdn.microsoft.com/Forums/sqlserver/en-US/9e6bb2de-8fd0-45de-ab02-d59bbe05f72e/servicedatabase-accounts-nt-servicemssqlserver-nt-servicesqlserveragent-what-are-they-for?forum=sqlsecurity