Grades in undergraduate science, technology, engineering, and mathematics (STEM) courses are distributed under the assumption that high-performing students have a strong understanding of the material. Similarly, the STEM education literature often presents exam performance as equivalent to understanding. Despite these assumptions, we have little knowledge regarding student thinking in relation to exam scores. To investigate this relationship, we asked undergraduate students to complete a series of written and verbal tasks. Twenty-two participants were presented with biology questions and were instructed to write exam-like responses along with their thought process. More than half of the participants then took part in retrospective interviews. We graded the exam-like responses to award a performance score and coded the entirety of participants' writing for their understanding. We found a discrepancy between performance and understanding for over one quarter of our data. Furthermore, interviews allowed for a more complete picture of participant understanding than written responses. These results contribute to calls for re-evaluating our course assessments and for questioning the understanding those assessments value.