Carlisle had (as was described in Book 1) really bad evidence. The way I'm cashing out Siobhan's power (which is confirmed in canon only by an asterisk next to her name in the Breaking Dawn character list, not by any actual achievement of hers during the book... for some reason there's not even a scene with her talking to Eleazar about it while they're in the same house...) is that when she makes a good plan based on good info, it will tend not to fall prey to black swan events and will tend to fall into the outcome she envisions rather than turning out in some wacky unexpected fashion.