Achievement tests and IQ/psychoeducational tests are entirely different, from what I understand.
So while your two children might score very differently on the former, they might score quite similarly on the latter.
And lower achievement scores do not necessarily translate to lower IQ. Apparently, gifted kids often do not achieve as well in school as you might think they should.
Our situation is a bit of a case in point: DS8 has tested as MG but his marks in school (grade 3) would really not be indicative of giftedness. By way of contrast, we haven't had DS6 tested but his results in grade 1 are excellent across the board and his teacher constantly lavishes praise on him. Age at admission seems to be rather irrelevant for both.
So in other words, it's hard to know just from the achievement tests. I think. But I'm just a parent with very limited knowledge in this respect (other than what I've read on here). Hopefully others will chime in.
