How might the "Imposter Syndrome" typology be reconciled with the robustness of the "self-serving bias"? TL;DR
What on Earth is going on in someone's head to invert a cognitive bias that is robustly observed in people who are psychologically healthy (Cohen's d = 1.28) and still present in depression (Cohen's d = 0.21)? I am looking to reconcile the self-serving bias with either the Imposter Syndrome or mental illness. 

"Some people", as the familiar trope notes, “are born on third base and go through life thinking they hit a triple.” While we are all familiar with individuals who are thoroughly unable to accurately attribute the true cause of their achievements, rarely do we ever consider the possibility that we too are guilty of the self-serving bias.

The self-serving bias appears to be both universal and robust. Moreover, an attenuated bias is associated with mental illness:



Mezulis, A. H., Abramson, L. Y., Hyde, J. S., &amp; Hankin, B. L. (2004). Is 
There a Universal Positivity Bias in Attributions? A Meta-Analytic   
Review of Individual, Developmental, and Cultural Differences in the 
Self-Serving Attributional Bias. Psychological Bulletin, 130(5), 711–747. 
https://doi.org/10.1037/0033-2909.130.5.711


Others, on the other hand, seem to be afflicted with an attribution bias antipodal to the self-serving bias. These people seem to completely unable  to internalise their achievements as truly their own, and appear to dismissively attribute their successes to external factors most people do for everyone but themselves -- the Imposter Syndrome. 

Clance and Imes (1978) first characterised the Imposter Syndrome typology among their high-achieving female patients:



Clance, P. R., &amp; Imes, S. A. (1978). The imposter phenomenon in high 
achieving women: Dynamics and therapeutic intervention. Psychotherapy: 
Theory, Research &amp; Practice, 15(3), 241–247. https://doi.org/10.1037/h0086006


So, how can we reconcile what seem to be two antipodal phenomena?

From a very cursory consideration of things, it seems that for individuals with the Imposter Syndrome, the self-serving bias is either greatly attenuated, or dare I say, inverted. For that to occur, we might presume two things:


They have crushingly low self-efficacy and/or; 
They are processing information in an aberrant manner. 


So, how does social, cognitive, or abnormal psychology reconcile the self-serving bias with the Imposter Syndrome? Is the Imposter Syndrome even a thing or simply a cognitive style that occurs among people with depression? If so, what are their similarities, and what are their departures? In other words, is the phenomenology of someone with severe depression effectively the same as someone with the Imposter Syndrome?

Where possible, please include references.
You might as well ask how comes there's depression (or low self-esteem in general) if everyone has self-serving bias. The answer is that the self-serving bias is reduced in depressed individuals; see Greenberg et al.(1992) or Alloy &amp; Abramson (1979) for original investigations; the latter paper has some 2000+ citations in Google Scholar. Allan et al. (2007) do note that there have been some successes and some failures replicating the Alloy &amp; Abramson experiments, so it's perhaps not a totally settled question. There's even a 2012 meta-analysis by Moore &amp; Fresco; quoting its abstract:


  The current investigation represents the first meta-analysis of the depressive realism literature. A search of this literature revealed 75 relevant studies representing 7305 participants from across the US and Canada, as well as from England, Spain, and Israel. Results generally indicated a small overall depressive realism effect (Cohen's d=-.07). Overall, however, both dysphoric/depressed individuals (d=.14) and nondysphoric/nondepressed individuals evidenced a substantial positive bias (d=.29), with this bias being larger in nondysphoric/nondepressed individuals. Examination of potential moderator variables indicated that studies lacking an objective standard of reality (d=-.15 versus -.03, for studies possessing such a standard) and that utilize self-report measures to measure symptoms of depression (d=.16 versus -.04, for studies which utilize structured interviews) were more likely to find depressive realism effects. Methodological paradigm was also found to influence whether results consistent with depressive realism were found (d's ranged from -.09 to .14).


When you study self-serving bias in the general population you might not get enough depressed subjects in your sample to notice the heterogeneity of self-serving bias, particularly since the difference doesn't seem big.



While I cannot give you any numbers on the change in self-serving bias in the impostor syndrome group relative to individuals unaffected by it (I could not find research on this specifically), I will point out some correlations between impostor syndrome and other psychometric measures linked to (depressive) psychopathology.

Bernard et al. (2010) looked at how impostor syndrome related to Big Five personality factors and found that Neuroticism correlated fairly well (0.52 or 0.47 depending on the impostor scale used; 0.52 on a combined measure) as did conscientiousness (-0.31 on both scales; -0.38 combined). In a hierarchical regression model:


  In Regression
  1, Neuroticism made a significant contribution accounting for 27% of the variance
  (p &lt; .001), whereas Conscientiousness contributed just 3% (p &lt; .01). When the order
  of entry was reversed in a second regression, Conscientiousness accounted for
  15% in a first step (p &lt; .001), but Neuroticism accounted for another 15% in a second
  step (p &lt; .001). As expected, the other three NEO–PI–R domain scales did not
  contribute to the prediction of IP, accounting for less than 1% of the variance. In
  other words, unconfounded by Conscientiousness, Neuroticism explained 15% of
  the variance of IP scores, whereas Conscientiousness unconfounded by
  Neuroticism explained 3%; the overlapping variance explained by both traits was
  thus 12%. In short, Neuroticism is the best predictor of IP scores, but Conscientiousness
  adds to the model.


Sonnak and Towell (2001) found a strong correlation of impostor syndrome with low self-esteem (on the Rosenberg scale) as well as with poorer mental health on the GHQ-12, which is mainly a measure of depressive symptoms. Correlated with the CIPS imposter scale, these two measures had r = -0.67 (self-esteem) and 0.33 (GHQ) respectively.