I had a woman at my work who had all four of her babies at 44 weeks. That's just how she baked (and she said she was definite on dates each time).

I think there is a large variation on normal, but our current model of medicalised birthing only works when things are standardized. It has also meant many mums and babies have been saved, but the cost to our society is a shrinking knowledge of things outside the standard and an inability to personalize care to match this variation. So yeah, I think some women cook babies longer, just like my periods were always 35 days apart.