Distinguishing easily confusable categories requires learners to detect their predictive differences. Interleaved sequences -- switching between categories -- help learners to detect such differences. Nonetheless, learners prefer to block -- switching within a category -- to detect commonalities. Across two 2 × 2-factorial experiments, we investigated why learners scarcely engage in interleaving when learning confusable categories. In Experiment 1 (N = 190), we investigated the role of the utility value of being able to distinguish confusable mushroom doubles on their spontaneous study sequence choices and of the conditional knowledge component that for distinguishing, the detection of differences (between the doubles) matters. In Experiment 2 (N = 134), we again investigated the role of the latter and additionally of the conditional knowledge component that interleaving highlights differences. Results showed that combining two factors -- increasing the utility value of distinguishing and informing learners that for distinguishing, the detection of differences matters -- fostered learners' use of interleaving. In conclusion, learners are more aware that interleaving highlights differences than previously thought. Nonetheless, learners prefer blocking because they do not recognize the utility value of distinguishing, and they lack the conditional knowledge that distinguishing requires finding predictive differences. Their blocked study sequence choices reflect a deliberate investment of effort to find commonalities rather than just avoiding effort. To make learners shift their effort allocation from finding commonalities to finding differences and engage them in spontaneous interleaving, we recommend highlighting the utility value of distinguishing and informing learners about the importance of finding differences for distinguishing.