A growing body of research suggests that the effects of core mathematics instruction on student mathematics outcomes may not be uniform across different skill levels in mathematics. This study investigated the extent to which observed components of explicit mathematics instruction explained why students' initial mathematics achievement was previously found to moderate the treatment impact of an empirically validated, core kindergarten mathematics program. Instructional components examined were: (1) teacher demonstrations and explanations of mathematical concepts; (2) group and individual student practice opportunities; and (3) teacher-delivered academic feedback. Findings suggest that the rate in which teachers facilitated individual student practice opportunities during core mathematics instruction explained the program's differential effectiveness. Implications in terms of differentiating practice opportunities for at-risk learners and utilizing classroom observation data to test potential mediating variables of academic interventions are discussed. [This paper was published in "Assessment for Effective Intervention" v44 n3 p197-211 Jun 2019.]