Because that is their previous login time. That information used to be private and was only shown when you log in, where it says what your previous login time was. I added that to profile, and thought that last login would be the good field, but I guess I was wrong. I want 'session_time', which I've switched it to now because that is the last known time they logged in.
And my test account is pretty damn obvious that it is me. Hell, the only posts with it is showing how long ago I last logged in with that account. Twice now.