Why @@servername Is Not Showing The Servername?

Hi

I have explicitly used
Exec SP_dropserver and
Exec SP_Addsserver SP's to drop and add a SQL server.

After that i restarted the sql server service. After that, when i used the query

select @@servername, it is returning NULL.
But, when I used Select Serverproprty('servername'), it is returning me the correct servername.
Can i know as why this behavior is exhibited for the same property of finding servername?

Thanks!