Due to the fact that we use vector format to render items sometimes borders might be rendered inconsistently, please check a note at the bottom of Design Considerations for Report Item Layout
GDI API that we utilize uses anti-aliasing, and depending on the width and the location of the reporting item, the lines may appear to be thicker or thinner (which is an unwanted effect in your case).
The general suggestion is to set the border with minimum 2px width which can be rendered on a machine with 96dpi resolution. Also using pixels instead of points as it is more media oriented unit.
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