There's something I always say about homeschool:
There's no behind in homeschool; there's only where your child is at.
In homeschool, we have the luxury of teaching the child on their level, at all times. We can advance at their pace. We can teach the same thing over and over. We can slow down. We can stop. We can let it go for months or years and pick it up when they are ready.
In general, a lot of homeschoolers play more in the younger years than their classmate counterparts do. It's not that hard to "catch up" later. Especially because one-on-one learning is so efficient.
In unschooling, where the parent doesn't teach the child, and instead, the child learns what they want when they want, there is often a fear that the child will grow up and blame the parent for not forcing them to learn. After all, learning is unpleasant but needs to be done, and you should have made me do it, even though I fought it. And now I'm an adult and I don't know what I need to know and it's all your fault.
(Sure, homeschoolers worry about this too. But believe me when I say the fear is a little more stark when you've actually actively not taught your children [unless they asked] as a philosophy.)
I've written about "You Should Have Taught me X" at length (and it's worth rereading).
The more experience I have as an unschooler and as a parent, the more I realize that the unschooling philosophy of education is a radically different method of educating and is going to look very different. That's why testing doesn't actually give very good information about where an unschooler is educationally.
Like if most kids (hahahaha as if) learn in a straight line, i.e. the older they get, the more math and reading they know
A Chinese bamboo tree takes five years to grow. It has to be watered and fertilized in the ground where it has been planted every day. It doesn't break through the ground for five years. After five years, once it breaks through the ground, it will grow 90 feet tall in five weeks!
My point is that since unschoolers learn when they want to or when they feel they need to, they can often go for years without what society deems basic scholastic competency. (Or they work around it.)
But it's a mistake to think that they are "behind." They are actually perfectly aligned with the unschooling educational philosophy. Which says that the time to learn is when the child (person) wants to or feels motivated to because they need it for something they want. This could happen after childhood, once the person is an adult.
I once heard my son explain: "Oh, you see, the way it works for us is that we learn it when we want to."
Unschooled children are never "behind." They are simply in a pre-state of "not knowing YET." And the happy state of "When I want to know it, I'll figure it out."