There are two possible things going with Sandra here: 1) Sandra accidentally knocked Rupert out of the challenge, then got so ticked off at Jon during the tribal council that she broke ranks and voted for him without considering the consequences for Rupert; 2) Sandra wanted Rupert gone, but knows she needs his jury vote in the final so she can't afford to make him mad; therefore, she knocks him out of the challenge and acts like she's made a mistake, then casts a vote for Jon in the tribal council to break any potential tie, knowing she could say that she just got angry at Jon's antics. Werent' there five votes for Rupert - Burton, Jon, Darrah, Lil and Tijuana? If so, there wouldn't have been any 4-4 tie then no matter how Sandra voted, but then she may have thought that Lil was in their camp and intended to vote for Darrah.
If I were playing this game I'd be keeping an eye on Sandra.