Rather than “my neighbor’s mansion,” why does the narrator tell us that the figure emerged from “the shadow” of it?in The Great Gatsby

It seems appropriate for us to peruse the text here before
we delve into this discussion.  Something very important to note is that this quotation
comes directly before we see Gatsby stretching his arms toward Daisy's green light at
the end of the dock for the very first time.

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Fifty feet away a figure had emerged from the
shadow of my neighbor's mansion and was standing with his hands in his pockets regarding
the silver pepper of the stars.  Something in his leisurely movements and the secure
position of his feet upon the lawn suggested that it was Mr. Gatsby himself, come out to
determine what share was his of our local
heavens.

Now, why does Nick
observe that Gatsby emerged from the shadows instead of from the mansion itself?  In my
opinion it is for two reasons:  to increase the air of mystery around Gatsby and to add
to Nick's gut feeling that he "disapproved of [Gatsby] from beginning to
end."

Give the novel another chapter or two and we will be
immersed in one of the "great" Gatsby's great parties.  Nick hasn't even met Gatsby yet,
and our narrator is suddenly drenched in Gatsby rumors that Gatsby has "killed a man,"
been a "German spy," was "educated at Oxford," buys disgruntled guests extravagant
clothes, etc.  This is all more about the Gatsby of the shadows:  the man of mystery. 
At his party, Gatsby is emerging out of the shadows again:  the shadows of
rumor.

Finally, our little Midwestern Nick, our little
unreliable narrator:  liking Gatsby one moment ("you're worth more than all of them put
together") and hating him the next ("because I disapproved of him from beginning to
end").  Most of the foreshadowing of this novel comes from Nick's very mouth, from that
first moment when he tells us at Tom's that his instinct was to "telephone immediately
for the police."  This Gatsby who appears out of the shadow in some ways remains in
shadow for the entire novel.  He remains the reason why Nick comes to the East (to check
out the shadows over here) and why he returns to the Midwest (to get the heck away from
these same shadows).  Gatsby's obsession and awkwardness and warped love.  Gatsby's
death and how the people of New York show their "respect" for Gatsby by not showing him
any respect at all.  These are all things that cause Nick to
flee.

From the beginning to the end, the great Gatsby
remains in the shadows: a purposefully ethereal image presented by the master of words,
Fitzgerald.