The aims of this study are (a) to examine the sources of differential functioning by gender via differential bundle functioning (DBF) in mathematics assessment and (b) to use DBF to explore whether the differential functioning displayed is construct-relevant or construct-irrelevant. Three qualitatively different areas, namely curriculum domains, test modality, and problem presentation, were investigated using the Roussos and Stout (1996) multidimensionality-based differential item functioning analysis paradigm with an adaptation of the Logistic Regression approach to model DBF. An empirical dataset from a mathematics national examination of 11-year-old pupils in England was analyzed. We argue that DBF found in curriculum domains may be construct-relevant of the measure, whereas problem presentation and test modality include arguably construct-irrelevant and so may indicate gender bias. (Contains 4 tables and 1 figure.)