It's been long since I watched this, but as far as I can remember, Saki's cantus "arrived late". I think it was in the scene where her bedroom's things begun flying around violently while she slept.
When she arrives at the academy, she says something along the lines of
Well, it's not so weird for somebody to get their cantus late, right?
But they tell her that nobody else got their cantus late. She was the only one.
Is it ever explained, perhaps in the novel, why did her cantus arrive late?