Howdy, some axles, Rubber Ride and/or leaf sprung are bent up in the middle in order to give you proper camber angle of the spindles.......or.......yours may have been jacked up in the middle by a previous owner, which could also cause the arch.....don`t know for sure!......I assume it`s usually easier for
axle manufacturers to weld the spindles in straight in a jig and then bend the complete
axle at the end of production....Benny