you formatting object yourself!
extract from the xsl-fo specs :
The child areas are placed such that the distance between the before-edge of the allocation-rectangle of the first child area and the before-edge of the content-rectangle of the reference-area is the same as the distance between the after-edge of the allocation-rectangle of the last child area and the after-edge of the content-rectangle of the reference-area.
…any person who’s able to understand this w/o making a drawing is a genius. The specs are full of this kind of crap :)