The individualisation of school mathematics teaching might be necessary in the face of the inhomogeneity of classrooms, but the choices, the students and the teacher face, might actually reinforce differences in achievement. In the following article I present and analyse data from a study about the emergence of disparity in achievement in mathematics classrooms. In the example we can see some of the effects of individualisation. The investigation shows differences in the teacher’s explanations on the same problem in the interaction with different students.