Averaging effect means that with the increasing number of segments with increasing radius, the differences in segment lengths average themselves out, so that the total segment length very closely approximates a simple segment count.
Averaging effect
Sources
Hillier, B., Turner, A., Yang, T., Park, H-T. (2007, 2010) Metric and topo-geometric properties of urban street networks: some convergencies, divergencies and new results. The Journal of Space Syntax, V(1) 2, 258-279. pp.263