I have been reading and digesting the document and found something interesting. Put in simple terms, the capacity is calculated by determining the amount of weigh that can be added to a hull with an engine and a full gas tank before it takes on water ( things like deck drains, scuppers, live well inputs and drains etc are sealed for the test)
Manufacturers now take that weight and adjust as they see fit to come up with capacity and max passengers. In general the specified capacities are less than the theoretical to give a safety factor. So, the amount of 'safety factor is up to the manufactures so two different models wit the same dimensions and molds could have different listed capacities as it seems is the case in the original post.
There is another factor though that I didn't think of. If the manufacturer states that the boat is unsinkable ( and for small hulls, it has to be), they need to calculate how much foam to add to account for the weight of the water that is flooding the hull. If there isn't enough room for the required foam, the manufacturer may opt to reduce the capacity. Secondly, in order to safe costs, the manufacturer may also reduce the capacity so that they can state that the boat is unsinkable.