DD had a couple of maths papers with unexpectedly low marks - as others have said, it was a combination of boredom with the whole process, and rushing because she wanted to do something else.
I would mark her papers immediately, and go through the mistakes with her. She would often say 'but that's what I put!' and explain her reasoning perfectly, so it was obviously careless answering rather than problems with understanding. We would go through and see how many marks she 'should' have had if she'd just been a bit more careful. This boosted her confidence, because she could see that it wasn't a "can't" issue, but something she could take control of. It also meant we could see where the genuine gaps were, and work on them.
I do remember worrying about it last year, but DD was much better at pacing herself by the time the real thing came around. And of course - in the real exam, there's no point rushing because you can't walk out early to do something more interesting
And she passed, with bells on - so in hindsight I would have panicked less.