Yeah, that's a big problem. I've worked with many UX people over the years, implementing their strongly held ideas and watching the metrics of user behavior afterward. My reluctant and painful conclusion is that they don't really understand users, it's all self deception. Though they do have good visual design skills.

Maybe the UX designer's job should include more statistics and analysis? It's not enough to draw mocks for engineers and do a usability test now and then.