No, it's not.
Here's 20 rolls: 10, 14, 15, 10, 17, 1, 13, 5, 13, 9, 20, 4, 1, 18, 12, 9, 3, 15, 6, 3. A +2 bonus would only affect the outcome of trying to hit DC 10 on 2 of those rolls like you said it would... but against a DC of 15 it affects the outcome of 3, and against a DC of 20 only...