well i think it s because the origin of your rhino file is in the same place as the origin of your archicad file and the slab you`ve built its maybe in the origin . The best practice is to work as close as possible to the 0 point (origin) in Archicad. Anyway, in your case i would just import a few elements in rhino from ac to see the position, or just use the GH connection to get some reference points of where you should build your GH slab, so its in the perfect position.
Good luck.