There are environmental effects. Common sense things can affect performance on IQ tests, including taking the test a second time in short succession (you get better with practice). Certain early childhood education interventions will cause children to over-perform on IQ tests relative to their IQ measured later in life. If the subject has the flu, they will also tend to underperform. The environment definitely plays a role, but the degree to which it is stable is, to me at least, surprising.