All the tap water in Australia is fluoridated. You would be considered a bit odd if you drink filtered water instead of tap water (bar say SA where the water tastes bad). All other things equal, we should expect 3&4 yearolds to have a 3.66 higher IQ than those, say, in NSW, where the water is treated and tastes good enough to drink?

Surely there are some population effects or something else going on here, or the effect, especially later on in life, evens out between the groups.