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Different software renders images differently especially if they are not being viewed at their native resolution. The image needs to be sampled or interpolated to be viewed, causing softening of the image. Some software will actually try to sharpen the image. And this can create sharpening artifacts.
That is one of the reasons why if your image is to be viewed at 800 x 640, you should sharpen it one last time at that specific size. Sharpening at a particular size whether for web viewing or printing should be one of the last steps after up / down rezzing the image.