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Staad: Master/Slave Joints

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Title: RE: OMF in low seismic zones
According to Staad's user manual, you can use the master/slave joints to model a rigid diaphragm.  Does anybody know its limitations/assumptions?
 
I'm playing with an example, and the results do not make much sense.  I made a 3d model for a 3 story structure.  I have moment frames evenly spaced in each direction.  I gave each frame a different stiffness.  Some frames are all W10's and others are W12 members.
 
Using the master/slave to make a rigid diaphragm, I would expect that each frame would have the same deflection, and the differences between each frame would be their stresses.  However, the results are showing each frame with varying deflections.
 
Anybody have any insight to this?
 
Is there a better way to make a 3d model?   (please don't say..."don't use Staad").
 
Thanks,
John Connor, PE
Kansas City, MO