An important component of many Adaptive Instructional Systems (AIS) is a 'Learner Model' intended to track student learning and predict future performance. Predictions from learner models are frequently used in combination with mastery criterion decision rules to make pedagogical decisions. Important aspects of learner models, such as learning rate and item difficulty, can be estimated from prior data. A critical function of AIS is to have students practice new content once the AIS predicts that they have 'mastered' current content or learned it to some criterion. For making this prediction, individual student parameters (e.g., for learning rate) are frequently unavailable due to having no prior data about a student, and thus population-level parameters or rules-of-thumb are typically applied instead. In this paper, we will argue and demonstrate via simulation and data analysis that even in best-case scenarios, learner models assuming equal learning rates for students will inevitably lead to systematic errors that result in suboptimal pedagogical decisions for most learners. This finding leads us to conclude that systematic errors should be expected, and mechanisms to adjust predictions to account for them should be included in AIS. We introduce two solutions that can adjust for student differences "online" in a running system: one that tracks systemic errors of the learner model (not the student) and adjusts accordingly, and a student-level performance adaptive feature. We demonstrate these solutions' efficacy and practicality on six large educational datasets and show that these features improved model accuracy in all tested datasets. [This is the online version of an article published in the "International Journal of Artificial Intelligence in Education."]