Why do modern estimates for clusters in the observable universe exceed historical counts?

Improved instruments shrink the minimum size classified as a cluster

The total count of galaxy clusters within the observable volume is a dynamic figure because detection technology continually evolves. As instruments achieve greater sensitivity and resolution, the minimum threshold defining what an astronomer classifies as a 'cluster' decreases. This means that structures previously deemed too faint or too small to register are now being included in modern surveys. Consequently, increased counts often reflect an improvement in the technological 'net' being cast into space—detecting fainter objects—rather than a genuine increase in the number of structures present across cosmic history.