My new drywall patch is thinner than the old one? Why?
Pro installers  will install drywall over irregular framing, which can give the impression of thicker sheets.  

Drywall can be skimcoated with setting joint compound.  This can be as thick as 1/4 inch, in some cases.  Originally, the skim was done with plaster

In any case, you  should fill with a setting compound mixed a bit stiffly and taper out 12 inches for a invisible joint.