Although many students struggle with fractions, students with mathematical learning disabilities (MLDs) experience pervasive difficulties because of neurological differences in how they process numerical information. These students make errors that are qualitatively different than their typically achieving and low-achieving peers. This study builds upon a quantitative study of fraction comparison errors and a qualitative study of students’ understandings to explore why students with MLDs make errors on the easiest fraction comparison problems. A detailed analysis of videotaped individual tutoring sessions with two adult students with MLDs revealed that both students understood mathematical representations in atypical ways, which may help explain the unique and persistent error patterns identified in students with MLDs. This study illustrates how building upon both quantitative and qualitative studies can provide a more nuanced understanding of student errors, which in turn can directly connect to implications for instructional interventions.