I would wildly speculate that someone said, “let’s make a SGR that is bluesy.” So they slowed it way down, but retained the underling current provided by F sharp, when playing in the key of E, to give the tune a power boost. While this is not peculiar to SGR alone, it wasn’t that common, either. Various other melody fragments are these, too. We’ll have to agree to disagree on this one, fellow guitar picker. :)