The difference is mostly down to the way each of us are using the term max weight.
We are quoting the max weight of the actual paddler. This takes into consideration the experience of a rider up to a particular weight. We make sure the standing area of a board is wide enough and of the right profile to handle a rider of the max weight while giving an authentic paddling experience. What many brands do is suggest the total weight a board can take including a passenger. What this doesn’t tell you is whether the board is suitable of someone of a certain weight as a solo paddler or how it will perform. Basically 1kg of body weight is equal to 1 litre of board volume so many 'max rider weights' simply indicate the point at which the board will start to sink. It’s another thing entirely to make a board with the right shape and profile to ride well.
Width is a key dimension as well. For example you could make a 16 foot board with a high volume but only 24” of width and it would be a nightmare to paddle and even worse with a passenger. Super unstable. So the wider the board the more stable it is and the better it can support a rider and a passenger. Our 10’8” Ride for example is great for riders up to 100kgs as it has the width in the right place. You could also add a small passenger to the board and it would still work well.
