The diagnostic accuracy and clinical benefits of instantaneous wave-free ratio (iFR) compared to fractional flow reserve (FFR) have been well-established in the literature. Despite the advantages of non-hyperemic pressure indices, approximately 20% of iFR and FFR measurements are discordant. Efforts have been made to establish the mechanisms as well as identify causative factors that lead to such a discordance. Recent studies have identified many factors of discordance including sex differences, age differences, bradycardia, coronary artery stenosis location, elevated left ventricular end-diastolic pressure, and diastolic dysfunction. Additionally, discordance secondary to coronary artery microcirculation dysfunction, as seen in diabetics and patients on hemodialysis, has sparked interest amongst experts. As more interventional cardiologists are utilizing iFR independent of FFR to guide percutaneous coronary intervention an emphasis has been placed on identifying factors leading to discordance. The aim of this review is to outline recent studies that have identified factors of FFR and iFR discordance.