They both did well on ACT (actually, they had the same composite - 25) but I know DS10 only finished half the science test, so I think he'll do better next year. My oldest 2 increased their composites 5 points or more between 7th & 8th grades, maybe just from knowing what to expect and better time managment (I can't believe they learned that much in a year).
Time management and stamina make a big difference I think. DS says that he is just so tired by the science test that he can feel himself dragging. When he takes the practice tests as a stand-alone, he gets about 4-5 points higher than on the real thing. I'm sure that is just fatigue.
On the issue of time management, DS told me that he didn't finish the reading test in part because one of the early passages was so interesting and well-written that he started to re-read it for the joy of it and then remembered he was taking a test and moved on. I wonder how many more questions he might have gotten through if he hadn't lost time lounging in the pleasure of a well-written passage!