Well, whoever wrote that has got two sentences mixed up together. It's been one of the biggest accomplishments in my career. (Though I prefer... It's been one of the greatest accomplishments OF my career.) One of the greatest accomplishments of my career has been passing the exam. You certainly do not need the "It's" and the "has been" in that one sentence. There are lots of ways to write that sentence, but that is not one of them! I think it had you really confused. But it is the writer's mistake. Well done for spotting there was something very wrong about it.

That's because the sentence is incorrect. Trust yourself and your English ability. We definitely are not perfect :)