One needs to understand that the slabs thicknesses are as per the amount of Bending moment it need to negate, however as per the IS codes the minimum thickness of slab is recommended as 125 mm or 12.5 cm.
The bending moment is result of the span for which the slab or beam may have been designed for.
Now coming to your Query that whether the slab thickness varies in the same floor level for residential building. Yes the slab thickness may vary on same floor all depending on the room sizes. If the Room size is big then the slab thickness may increase and if small room then minimum thickness shall be 125 mm as indicated by me above.
The slab thickness also varies due to its nature. Normally one way slabs are more economical as compared to two ways slabs, however based on the planning the and size of the rooms it is decided whether the slab can be one way or two way slab. If the ratio of room is 1: 2 or 1: 1.75 the slabs are normally one way, however if the ratio is 1:1.5 or less then it will be two slabs and in that case the slab thickness may increase.