...you'll do great. In my experience with big companies, requests like this typically DO have to do with a specific strength rather than an overall or holistic sort of expertise. It's rare for any company or superior to believe that one person could possibly transfer the entirety of her knowledge or skills to another, and mentorships are usually chosen so that one person, and hopefully both people, can offer the particular talent or skill that makes them more proficient in certain areas than others.
To quit babbling, probably your ability to solve certain problems in the past got noticed, they recognized that some of that ability was based in your experience in other departments, and they want to spread it around.
So...yah. Quit worryin, you'll be fantastic :)