STORY STIFFNESS

I would like to know how staad calculate story stiffness?

I am using FLOOR DIAPHRAGM and PRINT STORY STIFFNESS to get story stiffness from staad.

Now suppose structure with 5 floor and same height. All column and beam are of same size. When I check story stiffness result why they differ floor to floor when all floor are similar?

Kindly reply.

Regards,

PNSTORY STIFFNESS.std