It's also possible that both the comment and the code are right, and there's some non-obvious reason why +=2 has the effect of adding 1 here, and is the only wa

It's also possible that both the comment and the code are right, and there's some non-obvious reason why +=2 has the effect of adding 1 here, and is the only way to do it. (Not literally, as in this example, but something analogous.)