Correlation is not causation.
My two cents worth is that kids living in high flouridated(?) area's are likely to be inner city kids. Those that live out on farms have to learn independence and generally get out and about more, learn to experiment, make their own fun and generally give those brain cells a work out resulting in potentially higher IQ's! As well as other, shall we say less politically correct factors connected with IQ and poorer areas - including diet.