Mine was structured differently - no mention of rank (although like everyone else i'm guessing, I got the "we've ranked you" e-mail a while back), just that I had impressed them on interview day, and that they wanted to inform me that they thought i was a very strong fit for the school prior to decisions being released. The letter did not use the word "likely" either, but also did not mention anything about not being a guarantee of an accept, and had no real request for a response or follow up. Above was just stated in 4-5 sentences with a little more detail, and written by one of my interviewers.
I didn't apply to AZ so I have no frame of reference for comparison, but i think this might just be different interviewers wording things differently rather than them sending out carbon copy e-mails to everyone. Or at least that's what i'm going to pretend either way to prevent the temptation of obsessing about if
they sent different tiers of letters, and which type of letter represented what tier