Allocating students into separate classes within a school depending on their "ability" is common in many countries. This paper presents a theoretical discussion of the practice, considering why it persists despite a long history of research emphasizing consequential problems. Our discussion identifies and critiques four possible reasons that between-class ability grouping is being used in schools again today, despite research advising against the practice. These reasons are: 1) educators' perspectives that it facilitates differentiation and assists in managing the intensification of teaching; 2) advantages for students in "high ability" and "low ability" classes; 3) cultural acceptability; and 4) historical entrenchment, with little explicit policy direction guiding its use. Our critical discussion uses the Australian educational context as a case study. The example of Australia, where we live and work, is comparatively useful for other countries with similar practices, including New Zealand, Canada, the United States, England, and more. We argue that the reasons between-class ability grouping persists are based on misconceptions or agendas that are prioritized over equity and student educational outcomes.