many racks, in addition to having slots at the end for bolting on the two upper supports, will also have centerline boltholes for using a single stay down the center to your single hole on the frame.
online images of the topeak rack seem to show they have opted to place the topeak logo in this location instead
either find a rack that will mount center style
or drill the one you've got
or use 'p-clamps' to attach to your frame instead of using the pretapped mounting points