Yes, you are 100% correct.. however, there are cases where the following happens.
Unfortunately, there is always post-interview communication. Whether or not each party solicits it from eachother is another problem. The fact is, it happens.
Candidate has program Y ranked #2 or #3 on their list.
Candidate has contact with program Y. Program Y hears that the candidate is going to put them #1 on his/her list (through a phone call from a PD, email from candidate, whatever). Program Y bumps up candidate to a top rank spot. If the candidate had them only 2 or 3 on the list, program Y may have not done that.
Of course, the candidate can always lie, but they shouldn't, because it's wrong.
Candidate matches at Program Y.
I don't know whether this is wrong or right, ethically, but it most certainly happens. I do know that this is OKAY as far as match regulations are concerned. Programs want people who want to go to their program, and vica versa. It's hedging your bet, so to speak.