I don't think that early reading milestones can be reliably mapped to IQ.
A Google search, which brings up Davidson articles among others, shows that the two are linked. It's the other way that's not linked-- reading later doesn't mean a kid isn't gifted. Further, the earlier they spontaneously read (teach themselves), the higher their IQ.
Because there are so many variables that go into testing, if a test seems completely off, I think it probably is. My 2E/HG son has one really wonky test in his file. While he's generally 97th-98th, on one test, he was in the 20th. If that had been his first test, I think we would have been just as confused as the OP.
