randomness in blur, why is it different from the GIMP's blur?

Hi all
I am trying to blur a simple rounded rectangle with the
following command:

convert -size 150x150 xc:#F0F  -fill black -draw
'roundrectangle 29,29,
139,139,20,20' -blur 5x5 a.png

The rectangle is indeed blured but with some sort of
"randomness" or
speckels at the edges.
This does not happen when I use the Gimp's blur, everythig
is exact.

Anyone knows why this is, and how to achieve a blur like in
GIMP?

Thanks,
Meni

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