It’s indeed as @Gunther_Schwarze wrote:
To have consistent run widths and visible effects, these line types are translated into runs of lines and/or dots, which makes them nice, but slow. In addition, these lines or dots are translated to GDI+ objects for the preview, which makes them even nicer, but also even slower. Basically, they work fine when used as underlines for sums etc., however I wouldn’t recommend them as standard separator for tables.
If you’d like, feel free to add a request to the Idea Place category to add “normal” GDI drawn lines to the mix. The result may vary then hovever between printers, the preview and the export formats.