Etabs displacement ratios - Torsional amplification for seismic case

[yromanos]

Etabs provides a simple table in the summary output with "STORY MAXIMUM AND AVERAGE LATERAL DISPLACEMENTS".
This provides indication of maximum to average ratio to check torsional irregularity.

My question is how does etabs get these values. Does the software use all the nodes on a particular level? or does it chose extreme nodes.
When i manually output the displacement for a floor and check max and average i get values close to the etabs summary but not the same. any ideas?
what displacement nodes is etabs averaging and taking the maximum of?

thanks