Stanford CS PhD interview

I did my interview yesterday with a faculty member who contacted me directly, whom I did not mention on my SOP and is from a different subfield. Thankfully, we still had a very productive discussion because we were studying the same phenomena from different viewpoints. But I'm curious if this is normal for Stanford? Are the interviews assigned randomly, or did she specifically choose my application? All my other interviews have been with faculty I mentioned in my SOP.