take note that when A and B are mutually exclusive, then P(AUB)=P(A)+P(B). when they are mutually exclusive, none of the elements of A and B are the same. In one trial you either get something from A or from B, you cannot get a result that is from both A and B like the 3 just now. if they are mutually exclusive, something like "A is {1,2,3} and B is {3,4,5}" is impossible. Possible scenario is that it will be like "A is {1,2,3} and B is {4,5,6}".

when it is not mentioned to be mutually exclusive, you cannot assume that it is. like in this question