How staad is calculating centre of rigidity/stiffness of a floor in a building?

Eg. I consider the below building, beams and columns size are 300x300mm. In addition 2kn/m2 floor load is considered on all floors.

Using "Print Dia CR" command , staad is showing the below result

How the rigidity is changing on each floor as the beams,column & loads applied is same?

What are the parameters considered for calculating stiffness?Usually if we are doing manually, stiffness of column alone will be used for calculating the stiffness/rigidity on each floors.

Please explain how the above calculation has been made.

(Above Example is taken from STAAD.Pro CE – Seismic Analysis Using IS 1893 (Part 1):2016 problem statement 10.1)