How might the "Imposter Syndrome" typology be reconciled with the robustness of the "self-serving bias"?
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.