If the 64-bit JRE is installed, then double-clicking on a .jar file will use that JRE even if the 32-bit JRE is also installed. So if you install the following on your system:

32-bit JRE
64-bit JRE

32-bit FX runtime

Then double-clicking on an FX application jar file will not find the installed runtime. The standalone launcher will then attempt to load the runtime using a relative path, and in the case that one is found (which it will do if running the apps from the unzipped SDK), that will fail because it will attempt to load a 32-bit DLL with a 64-bit JVM.

Further, because of RT-13602 it will just silently fail with no error message (and the javaw process will be left lying around).