I think I know why his boat/trailer is only 5520 lb. That's the boat/trailer weight hitched on the towbar he probably has 500+lb on the towbar. The vehicles weight 6480 lb, I'm not familiar with the Canyon's weight but it should be a lot less than that unless he had a big load .
The weight I specified was off the towbar. Hooked up on the LC she was only 6300 lb ( approximately ). I had the combo weighed on a public weighbridge, they are very accurate. They are very strict with weights here, sometimes they set up near a boat ramp and weigh all the boat/trailers that's the reason I weigh mine so I know I will not go over. My trailer is rated 7000 lb, at times I'm very close to that.
EOH brakes are not optional here, they are a must to register any trailer over 2000kg (4400lb).
Mustang, the later model boats, with the full aluminum rear swim platform and longer bracket weigh a bit more, the transom has more reinforcement probably to cater for the extra HP rating. I know GW say the same on their website . I don't think you will find any two 228's that will weigh exactly the same, there are a lot of variables on boats.
The older 3.3 engines are a bit heavier as well.
I think KY has the best tug!