Some homeschooling families don't learn subjects in the same order that public school children learn them. They'll do a "unit study" and cover a broad range of topics all at once. Some families prefer to go at the child's pace (i.e. don't give them algebra if they don't understand basic math).
If that's the case with this family, the state may jump on a slightly low area and say, "These children need our help!" The children would then be stuck in the public school system wherever the state wanted to put them. Unlike their public school peers who are automatically moved with their class no matter what their grade is.
I homeschooled my kids for years and helped teach a number of other kids at home as well. I soon gave up doing regular "curricula" - I am not organized enough, didn't have enough time or money. I taught in a very "organic" way - let the kids learn at their own pace, invented lessons and tests, etc. Result? When my kids had to go to school for a while when I had to work full time, they were in the top 5% and had to get bumped up to "talented and gifted" classes. And I was a high school dropout.