The marker is retrieved and shown below a horizontal rule in the header
area. But for some odd reason there is TOO MUCH SPACE between the Black
Horizontal Rule and the running header retrieved as a marker.

You have a 1mm padding in the table cell you use for the ruler
(why no using fo:leader for this task?), and another 4mm top
padding on the block in the marker, for a total of 5mm. The
actual text will also cause another small space (the line half
leading). So: how much is too much?

Furthermore, it would help if you stated which FO processor you
are using.