I suspect that's a rating for the top box adaptor fixings rather than the rack, which has its own rating which is maybe listed in the owners manual (can't recall exactly, but sure I've seen a rating for it somewhere - 10Kg maybe). I fitted an SW-Motech adaptor plate with Givi mountings to my 1050XT and that's only rated at 3 or 5Kg, from memory, whereas Givi's own plate is rated higher.