lightweight steel framing system

huangyhg

lightweight steel framing system
i will use lightweight joist construction for a residencial project.  since my joists are 12" o.c.  i am using a form deck ufs 9/16" thick and 2 1/2" slab. this gives a capacity of 480psf for a single span.   however i have seen residential construction where for the same application people are using 1 1/2" deck plus 4" slab.  is there a reason for this?  
fire rating?
other reasons could be that the supports are spaced farther apart (more than 12 inches on center). another possiblity, is the required floor loading is higher.
sound?
floor vibrations are quite a bit less with the higher mass 4" slab thickness.
thanks all for your replies