To me, it's two things:
1) Close to 100% chance to get in
2) Willing to go there if the others say no
Edit: Tonights, it's amazing how similar our situations are! My undergrad institution is one safety, for all the reasons you gave. I have another "double-safety", in my home state, which is also where First Choice is. If FC says no, and something happens where my family really needs to move to Home State, then I'll attend Double Safety, which is substantially less selective than my qualifications, but has a nice department with good people.