Yeah, when I saw that last night, I figured that Survivor would've been changed in terms of the air date. I was too lazy/tired last night to further investigate, however. Given the number of teams left, I can't see anything else other then TPTB skipping a week or a recap show. Per Survivor's list on TFC, they have every episode listed as a real episode, even though one of them is slated to be the patented clip show.
Given the number of people left in Survivor Samoa, it does not look (super) realistic that Survivor will wrap up on the 13th of December. Remember that they typically do a recap episode on Thanksgiving, so that's one day gone. As a result, it's looking like Survivor will end on 12/20. Of course, that technically shouldn't affect whether TAR should end on the 6th or the 13th, but since the move to Sunday, every season (save for TAR12) ended the week prior to the Survivor finale and live reunion. As mentioned, it's probably due to media reasons.
Yeah, Survivor ending that late is not normal, as it's close to the holidays. However, Christmas is at the end of that week, so I guess it shouldn't be that big of a deal.