What would you read into it? That he's going to rank you to match? There may be plenty of game playing to come, but it's not clear to me what "reading into it" would make you do differently. Would his thank-you note make you think he's ranking you to match, thereby causing you to rank the program higher than you might ordinarily?
Like surfer said, look at it as a sign that the guy is a decent human being. Maybe he sends them to everyone, and maybe he doesn't. You'll never know whether or not this makes you more special than the other 200 interviewees, so I would stop worrying about.
This topic, in various forms, comes up every year. "Chair at such-and-such program told me he's going to buy me a car if I rank him #1. Does this mean anything?" I actually always wonder if this isn't secretly an attempt on the part of the poster to garner admiration from the masses.
But ultimately, none of the communication back and forth, the promises they make and those they try to elicit from you, mean anything. Ultimately, your job is simple: rank the programs in the order you would prefer to go to them, irrespective of wherever you think they are ranking you. With this perspective, you will place their thank you notes and other things in the proper context.