Set the LayoutRounding property to true. This happens when after layout calculation a position cannot directly mapped to a screen pixel, and the color value bleeds into two display pixels. Consider a horizontal line placed on y=3, which would fill that line with a complete color, if it would be placed on y=3.5 it would need to half fill the line on 3 and half on 4. But there aren't half pixel obviously, thats why its blends the color between the line. So you usually get a half transparent line on y 3 and 4.
Sometimes this is desired especially with animations, because this sub pixel rendering makes everything look a little bit smoother, but for icons this is usually undesirable.