That's an interesting point. A classmate of mine got a call a few weeks ago she described as going like this (location changed for privacy purposes):
PD: "I just wanted to call you to let you know that we're very interested in you, and if you choose to rank us #1, we would expect to see you here next year."
Student: "Well thank you. I really liked your program a lot, and you'll definitely be very high on my list."
PD: "So . . . does that mean we can expect you to be moving to Miami soon?"
Student: ". . . Yes. Yes it does."
PD: "Great, then we'll see you next year!"
She was a little confused about the call, and wasn't sure if the PD was fishing for a firm committment to rank them #1. Fortunately for her, this was her first choice program. The whole thing lent credence in my mind to the idea that the programs might "pad" their choices this way to ensure that they don't have to rank too far down the list.