I think it may have to do with agency and/or clearance level as well.
A couple of years ago I was contacted at work from an investigator who was investigating a friend/colleague of mine who was put in to get upgraded to TS (DoD agency, non-IC). I was one of his references on his SF-86. The interview was done completely over the phone and lasted maybe 5 minutes.
Last year, I was put in for TS/SCI from an IC agency that I accepted a CJO from. The investigator drove 3 hours from where he was based to come on the installation I currently work at to interview me and 5 of my references. He was able to get on the main part of base by himself but I had to escort him into my building and reserve conference rooms so he could interview each of us privately in person.