Master and slave nodes have different displacement ! is that possible? and why?

I defined master and slave nodes to connect two structures, one supported the other; so the M/S is the link between the two structures. why after running STAAD I got different displacement at these linked nodes? is there a better approach to model the link between these two structures, since I have notice the result stresses are different in members than the result of the two separate structures when they were modeled separately.