This is probably just the noise (i.e. things like "blind luck") being averaged out.

And another plausible explanation is added to the list...

Heritability studies tend to be done on people living in the same country, of roughly the same age, which means that population-wide effects like the Flynn effect don't register.

Oh, I understand why this is the case. It just means that the outcome of many changes (if they are country-wide) are hard to estimate (and are typically underestimated from twin studies).