Session vs HttpContext.Current.Session?

I've been reading that to store information thru session. I can
use either the following statement in the code behind:
    Session["ID"] = "12345";  
OR
    HttpContext.Current.Session["ID"] = "12345";

But it didnt work so I have to use both as follow:
    Session["ID"] = "12345";
    HttpContext.Current.Session["LoginSuccess"] = Session["ID"];

I never got a clear understanding of why I need to use both statements.
Can anyone clarify this for me?
Thanks.
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