This study focuses on the perception of numerosity of item sets placed in retail displays. Previous studies have demonstrated that the item sets’ perceived numerosity decreases as the number of polygonal shapes placed in a panel behind a display increases. Such a result was explained by a non-spatial clustering phenomenon exerted by the shapes. Our research reveals the perceptual mechanisms underlying the described effect. Using an eye-tracking procedure, we highlight that upon augmenting the number of polygonal shapes in the decision-making context: (a) there is a significant decrease in the number of total fixations per display; (b) there is an underestimation of the perceived numerosity of item sets involved. The findings suggest that the mere presence of visual shapes can alter perception generating complex objects or “super-items”, which tend to perceptually replace entire item sets. We also propose managerial implications in terms of category management and merchandising.