They aren't assuming anything. They know that healthcare in the US is hit-or-miss, that it is normally for people to move between having and not having healthcare throughout a lifetime.
>> The new report highlights that 5.5% of children under the age of 19 were uninsured, largely because of a decline in public coverage.
https://www.census.gov/library/stories/2019/09/uninsured-rat...
1 in 20 American kids are uninsured, not counting the undocumented who don't answer the census. That's ridiculous today but it was even worse in the 90s. That's why the Chinese didn't want to give kids to non-wealthy Americans. Uninsured children are almost unheard of in Canada. Only some illegal immigrant children, but even there they have systems in place.