Black points with statistical errors: background-subtracted and completeness- corrected UDGs in the ensemble cluster (as in Fig. 8). Grey area: the systematic error due to field-to-field variance in the reference background fields. Dotted curve: best-fit projected generalized NFW profile to the stellar mass distribution of the total population of quiescent galaxies in the clusters (vdB15), rescaled in normalisation. Black curve: same but with an excised core of 0.15 × R₂₀₀ (before projection).