You have that backward.
The home-educated typically score 15 to 25 percentile points above public-school students on standardized academic achievement tests.
-- https://nheri.org/research-facts-on-homeschooling/#Academic
The home-educated typically score 15 to 25 percentile points above public-school students on standardized academic achievement tests.
-- https://nheri.org/research-facts-on-homeschooling/#Academic
In all three scenarios (but especially in the first two) there is a sizeable population of parents who know their kid is several years behind grade level, but they tolerate it because they think kids learn at different paces. Or that it's fine for a kid to love one particular thing (cooking, machine repair, art) at the expense of a well-rounded education. And that's not even getting into the 'unschoolers' who treat never opening a textbook as a point of pride.
If you're well-educated and run in circles of people like you then you'll find the homeschoolers who also value education. But if you expose yourself to a wider variety of people your conclusions may change.
First independent search result:
> The US-based NHERI describes itself as a leading research institute in the field of homeschooling. However, its neutrality is often questioned.