This study investigates intraindividual variability (IIV) in curriculum-based measurements of writing (CBM-W) among primary school children. Inspired by the idea that such fluctuations may not only represent measurement error but also warrant description in relation to writing performance and development, the study examines patterns of IIV in this context. Data were collected from 345 children (51.6% female) in Grades 3–6 in Switzerland (mean age 10;5), including both monolinguals and multilinguals learning German. Students produced 10 writing samples at two time points (fall, spring), scored for correct writing sequences. The coefficient of variation quantified IIV at both times. Results show that IIV is systematically associated with performance level, with higher variability among lower-performing students. No clear age-related pattern was found once performance level was taken into account. IIV did not emerge as a predictor of subsequent writing development among students with comparable initial performance. In addition, students’ home language background did not moderate the association between IIV and subsequent writing development. These findings suggest that IIV in CBM-W is closely related to performance level and highlight the importance of interpreting variability in relation to students’ performance, calling for caution when using CBM-W in educational decision-making.