How often do Ph.D. holders hold faculty positions in different, but related, fields from their Ph.D.?
For interdisciplinary researchers, this sort of thing can happen fairly often.  For example, if you are a chemist working on ecological applications, you might well have started out in a chemistry department and end up in an ecology department, or vice versa.  This happens even more so when dealing with disciplines that may be categorized differently by different institutions: for example, is work on medical devices part of biology and the natural sciences faculty or part of electrical or mechanical engineering?  Generally, however, if it's going to happen, there is at least some logical trail that you can see how the faculty member got from point A to point B.