Don't read too much into these stats. While it does show Green has performed admirably when he has played against James, these stats don't show what goes on when James doesn't shoot the ball. Leonard has been guarding James for a much higher number of minutes than Green. Just from my on personal eye test, what I think is happening is that James has been forcing himself on offense whenever he has Green on him and is taking bad shots. If Green ended up taking primary responsibility on James I think he would figure him out and do much better.
Again I don't want to discredit what Green is doing out there, but I just don't think these stats really tell all that much.