Story Drift Calculation

I see STAAD offers a feature to calculate the inter-story drift. I have some questions as following:
1. STAAD automatically identifies floor without any input and the story drift is seemingly calculated based on the centre of the floor. How can it be done? Please explain the calculation behind the results.
2. STAAD produces the same story drift results regardless the input of floor diaphragm. As node displacements are different, I supposed the results should be accordingly different. Please explain.
Thank you in advance.
Floor-Diaphragm.STD
No-Floor-Diaphragm.STD