The difference would likely be explainable by the accumulative differences between constant stimulation and not being stimulated.
If you took two people, say twins, who were largely identical in every meaningful way, and gave them a differing level of challenges, over many years, the person who led a more challenging life will probably have a measurable higher IQ difference than the person who wasn’t challenged sufficiently.
When we send our kids to school, based on the current curriculum and method of teachings, you are sending boys to check out out for 5 hours a day and girls to check in for 5 hours a day. That has to create a difference over time.
I don’t disagree with what you’re saying, but the concept that environment, including educational environment, significantly affects IQ scores is contrary to the notion, popular with some FReepers, that IQ is set in stone at birth and that large swathes of the population are inherently incapable of learning to read and do basic math.
Someone ... maybe the Thernstroms, or Thomas Sowell ... wrote about a study which showed that IQ in elementary-school age correlates closely with the child’s aural vocabulary - the number of spoke words he understands - at age 5. The writer pointed out that talking to and reading to a young child is a very low-cost, high-return intervention.