I think it's important to realize that not all kids who score at very high levels on such tests have been test prepped or formally taught higher level material. To me, that belief feels a bit like what the schools always tell me -- that it can't be possible for my kids to know what they know unless I'm some crazy nut making my kids do math 3 hours a day.
I'm sorry if my post came across this way - I absolutely did NOT mean that. I would consider "exposure," though, to include self teaching, out of self desire. Heck, if the SAT tested train knowledge, my son would score 800, and it bores dh and me to tears. 
I confess that I always found the Explore scores taken as DYS levels to be low. I suspect that the group taking such tests are not in the top 5% as expected (most talent searches require parent agreement that their children had some subscore in the top 5%), but rather something like top 1/3 or 1/4 of ability tests.
I don't have a strong opinion on this. Both my kids who took EXPLORE had no problem exceeding-with room to spare-DYS levels. OTOH, the one child whom we have had tested and for whom we at least have test data showing to be PG+ is not one that I imagine would have scored particularly highly on EXPLORE. She was never interested in sitting at a computer or even sitting with books. She was very active, very artsy, very musical as a younger child (she now works as a research analyst, so she did become more focused on traditional academics). The IQ is there and math and science have come quite easily to her, but she certainly had no interest in self-learning it while in elementary.
I am also curious why your son would take the ACT very early while in elementary. To me, that does show parent interest/motivation of some sort, since it's unusual to take it even in middle school, and most kids can't sign up on their own. Again, I'm not saying that's bad in any way and I'm not suggesting that you are pushing. I guess I'm just wondering about the changes in Talent Search over the past MANY years and how all the resources out there (for self study, classes, enrichment, etc) have changed things.
I don't think that this is bad AT ALL - I'm truly just curious. I'm comparing three different groups of gifted kids- my own generation, my oldest kids (now adults) and my younger ones (elementary/middle). Each group has had many more options for learning, including self learning (my older kids didn't have Khan Academy, Art of Problem Solving, or even a home computer or TV when they were young). It honestly started as more of a philosophical musing... not as an accusation against any parent. 
