pressure calculation

Dear all,
   In order to computing pressure of specified region in my heterogeneous system,I use this command-"compute stress/atom".But when I calculate the pressure of purewater system,I find that the results which obtain from "compute stress/atom" and "compute pressure"(without kspace) is different. I cann't understand this problem!
   Best Regards,

If you are sure you are doing the comparison between
the 2 methods the way it is described on the compute stress/atom
doc page, then please post a simple example input script
that calculates the 2 methods for a single snapshot (e.g. a data
file) of your simulation and gets a different answer.