Electrons can't read assembly manuals. They go wherever there's the least amount of resistance. Since the heaviest current draw item on the truck is the starter, it makes sense to attach a heavy ground cable as close to the starter housing as possible and use smaller-gauge ground wires/straps to ground the body, frame, and bed to the engine.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!