I scored above the 94 percentile in all categories on the sat and our family had very modest means.
My brother scored 99 in about 4 categories
Another way (besides just pure luck) that two below average intelligence people could produce above average intelligence children is this:
A man with (overall) low intelligence is none the less good at math mates with a woman of (overall) low intelligence is none the less good at verbal skills. If their children were lucky, they might get his great math skills and her great verbal skills.
I see this in my own children, where some of my four children lucked out and got the best of both of our strengths, while others more closely mirror me or my wife.