Take a look at the behaviour in JLS - Compound Assignment Operator. I'll quote the relevant two paragraphs here, just for the sake of completeness of the answer:
Otherwise, the value of the left-hand operand is saved and then the right-hand operand is evaluated. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason and no assignment occurs.
Otherwise, the saved value of the left-hand variable and the value of the right-hand operand are used to perform the binary operation indicated by the compound assignment operator. If this operation completes abruptly, then the assignment expression completes abruptly for the same reason and no assignment occurs.
Emphasis mine.
So, the left hand operand is evaluated first, and it is done only once. And then, the evaluated value of left hand operand, 1
in your case, is added with the result of right hand operand, which turns out to be 5
. Hence the result 6
.