I think you daughter’s experience is more typical than your son’s in that scores on the actual test or a mock full test tend to be lower than what you get at home or when doing one section…we saw this on the SAT English when DS took it. It seems the SAT math questions are getting a bit easier but the curve is harsh and it is hard to not make sillies in that environment.
DS has competed in math competitions for years and silly mistakes have plagued him the whole time. He qualified for the AIME in 6th grade and then missed qualifying again until 10th grade due to silly mistakes on the AMCs…in tenth he barely qualified for the AIME and then scored a 10 on it, which is a very high score and the questions are quite a bit more difficult than the AMCs, but there is more time and it is not quite as fast-paced so he did not feel the pressure as much.
Rechecking your work is often not too helpful in finding mistakes but that is about the only thing she can do…that and try to relax as discussed. If she is a very strong math student and knows several ways to solve the problem, sometimes it can flush out silly mistakes to solve the problem a different way to see if you come up with the same answer. Good luck to your DD!!