It is prob because what is sold as 8mm is not 8mm...it is 8.1 or 8.2 etc, and the depth of the stones will be different. Also, the moissy just has a different cut...and your setting was *made for* a different stone.
I like the face down view as well, but the side view looks a bit like the stone doesn't fit well.
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Joseph Schubach Jewelers