The noise you are adding is statistical, not identical on every run. The way it will affect the signal is going to be statistical, not identical. The noise removal is never going to be perfect. You should expect small differences in the output.
Like suppose your filter was effective in removing 99% of the noise. That would still leave 1%, and that 1% is not going to be acting in the same place and same magnitude each time, so a difference in output would be expected.
