Here's what I've noticed from more than 20 years of helping to prepare colleagues' promotion cases.
Many are excellent at both teaching and research. It is not at all unusual. There is a folk belief that abilities / accomplishments are inversely related. Let me suggest some reasons why this isn't the case. (Of course, I would never deny that it could be the case in a subset of people. That some people do not manage to to both well.)
1. Capable, talented people are ... capable. The ability of someone to one of their job tasks well may correlate with their general ability to do things well. By the same token, people who are not capable in general tend to have many things going wrong at once. So badness in one area is not a predictor of goodness at something else. "Oh, you are bad at sports, you must be good at math." I guess there is a folk belief to the contrary, a kind of equilibrium in which smart people aren't pretty, etc... That says more about our dichotomous thinking than about the real state of things. For example, my sister was the musician, so I kept my accomplishments modest. Maybe she is an 85 in musical talent, and I am a 60, but that is just a difference in degree, not a dichotomy between the musical and the unmusical.
2. Intellectual curiosity, knowledge of the field, self-confidence, a strong work-ethic, etc... There are several traits that underlie excellence in both areas.
3. The best people I have seen (and I'm not including myself in this) seem to be able to do much more than other people. They have more books and articles and are also capable administrators and mentors. They just manage to do more while complaining less about it. They often don't exude the typical academic negativity (again, this is not me, always).