Stacked δ distribution for the 17 most massive haloes, each analysed across three independent projections. The upper panel shows the normalised count of δ values, while the lower panel displays the ratio of each model relative to the CDM baseline. The FCT model, particularly when including only subhaloes with M_h > 10⁹ M_⊙ (FCT > 10⁹), exhibits a clear excess at high δ values compared to other models. This result supports the potential for detecting PBH-induced substructure through brightness fluctuations.