@nprofile1q... it is not a simple bolt; there is an unthreaded rod used to allow one end of a structure to be fixed and the other to float. A load slid out 12" on a shaft is going to lever, so I'm guessing the advertised rating is testing at least the midpoint on the shaft.

The large diameter is to disperse force. Most of the failure will be crushing the wood beneath the screw, not failure of the screw.