I have no psychology experience but after reading the wikipedia page of Impostor Syndrome, Dunning-Kruger effect, and skimming through Dunning and Kruger's Nobel Prize paper[1] there is a third effect/syndrome that seems to be involved: False consensus effect [2]. They state: "Simply, put, these participants assumed that because they performed so well, their peers must have performed well likewise."[1]. If you are interested as how they reached that conclusion refer to section "Burden of Expertise" on page 1131 (pdf page 11) in [1].
As an extra note: The Impostor syndrome is not an officially recognized psychological disorder[3]. I've seen the Impostor syndrome mentioned many times but had never taken time to read the complete wikipedia page.
http://en.wikipedia.org/wiki/Impostor_syndrome