There are several possibilities for the differences you are seeing.
- the version of HYSPLIT may be different….we test new updates to HYSPLIT on the web site before releasing them to the PC version,
- the values we set in the SETUP.CFG and CONTROL files may be different than what you have set in yours,
- finally, due to the random number generator, the dispersion will be very slightly different when running the same case on different computing platforms.