I would guess it has something to do with the fact that the outermost portion of the wing is actually an extension "tacked" onto the original wing design. IIRC, the -4XX 747s have an extra two metres of span (per wing), in addition to the winglets themselves.
To me, it appears that (in flight at least), that last outer portion is actually even (0 degrees to level), whereas the rest of the wing has a dihedral form, giving the illusion of a "droop."
That's my take on it anyway; and worth exactly what you paid for it,
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