1) Consider that Maison's scale has some level of uncertainty, by a 1/10 of a kg. And that uncertainty can be expressed this way:
Since the level of uncertainty is given in tenths of kilograms, and we don't know whether this will increase or decrease the weight of Mason's dog, a good approximation would be to express this way.
2) Therefore, Mason can express the weight of his dog as:
Since this uncertainty can increase or decrease the weight of his dog.