The atmospheric dispersion corrector (ADC) of the Gemini Planet Imager (GPI) corrects the chromatic dispersion caused by differential atmospheric refraction (DAR), making it an important optic for exoplanet observation. Despite requiring <5 mas of residual DAR to avoid potentially affecting the coronagraph, the GPI ADC averages ∼7 and ∼11 mas of residual DAR in H and J band, respectively. We analyzed GPI data in those bands to find explanations for the underperformance. We found the model GPI uses to predict DAR underestimates humidity’s impact on incident DAR, causing on average a 0.54 mas increase in H band residual DAR. Additionally, the GPI ADC consistently undercorrects in H band by about 7 mas, causing almost all the H band residual DAR. J band does not have such an offset. Perpendicular dispersion induced by the GPI ADC, potentially from a misalignment in the prisms’ relative orientation, causes 86% of the residual DAR in J band. Correcting these issues could reduce residual DAR, thereby improving exoplanet detection. We also made an approximation for the index of refraction of air from 0.7 to 1.36 microns that more accurately accounts for the effects of humidity.