Don't spend your time worrying about why your calcs don't match up exactly to the OE payload rating. It's not a big deal and you may never figure out the discrepancy.
By the rating, 4 large adults and a big dog in the back would overload it. Does that seem plausible?
Worry about tow rating and rear axle rating (assuming you're worried about your trailer weight and not just a hypothetical question).
At that, imagine the factors of safety designed into the vehicle components and don't obsess about a few......hundred lbs here or there.