In previous work, we demonstrated the existence of differences between students' epistemologies (beliefs about knowing and learning) in classical and quantum physics. Here, we explore the pervasiveness of these "splits" using five bifurcated items from the Colorado Learning Attitudes about Science Survey (CLASS) distributed across ten courses and four institutions (N=571). Data come from both lower and upper division quantum mechanics courses, with a variety of instructional and pedagogical contexts. Further, we investigate why students answer questions differently about classical and quantum physics through analysis of focus groups. We find splits in all course-instances surveyed. We develop a coding scheme that documents variations in epistemic beliefs between classical and quantum along themes of the following 5 categories: intuition, mathematics, social/cultural/experiential, quantum weirdness, and coherence. Lastly, we provide instances where students' epistemological splits reflect a sophisticated stance towards learning.