I've been told this as well, and one of the PDs explained why he wanted me to contact them in January if I was interested.
They do interview about 10 times the needed number of interns, and many of the less desirable programs know that they are not the Ivy league places that we all would love to match at. Therefore, if they like you as a candidate (and maybe think you would be competitive at other places that are more desirable), then they will only rank you if they know you would actually want go there. If not, they would rather rank another applicant who actually do want to go to their program.
One place I interviewed at told me that I would fit in perfectly, and that I would have no problem matching. At the end of the day the PD told me this again, and then said I should email him in January to let him know if I was planning to rank them high/top. If he knew that I ranked them as my #1, he would rank me to match. If not, I would go down on their rank-list, as he thought I would match at another place instead, and he would rather rank another desirable candidate higher.
If this is true or not is another story - you never know. But I don't think it means that you are their top choice, just that they think that you would fit in well with their program and that they would be happy to have you as an intern.
My advice would be to email the PD in January telling them that you plan to rank them highly (not no 1, as this is not your intention), so they will know that you still plan to rank them. Then it's up to them to decide whether they believe they have a chance of getting you or not...