The problem with homeschoolers using standard classroom Math textbooks is that the teaching of the new concepts generally isn’t included in the textbook. The textbook’s purpose is just to provide practice problems once the concept has been taught by the teacher. School textbooks aren’t designed to be self-instructive.
There’s two problems I’ve seen happen when homeschoolers use school textbooks, particularly for Math. One – the parent hands the book to the child and leaves them to it, particularly if the child can already read the instructions for themselves. This skips the teaching and just expects learning. Or two – the parent explains the concepts based on their own knowledge. This works fine in grades 1 to 3, and some topics in the upper grades are pretty easy too, but there usually comes a point, more quickly for some than others, when you are out of your depth and spending ages googling your child’s math or reasoning that it’s time to put them in school. The problem isn’t the parent (or the child), if they are motivated, it’s the books that are being incorrectly used.
You need solid teaching or the learning doesn’t happen as it should.
I’m not saying that you have to rush out and buy the teacher books to go with your school textbooks (they are incredibly expensive compared to the textbooks, because they are designed to be purchased by schools with big budgets). Teacher books, designed for schools, might not even include the teaching. It could just be the answers. I’m not sure. In the classroom, I never used one. I taught the same few new concepts every year. I just had to work out how to teach those few things during my first year in that grade level, and then I was set for the years to come. (The only thing that’s really hard about teaching in a classroom is the behaviour management and politics. Teaching is the easy bit!)
As a homeschooling parent, I regularly use the teaching guide that goes with my Math curriculum. Every day I’m covering new ground, ground that has got awful rusty over the years, and I need the support of a guide to fill me in quickly with the best approach to the day’s teaching. This is, in part, why I specifically chose a homeschool Math curriculum. They are designed to help you with the teaching process. The option isn’t as cheap or as easily accessible as a school textbook, but when you factor in the cost of the teaching support, it’s a fair price (and definitely cheaper than a school textbook with their exorbitantly priced teacher guides!).
Just something to consider when selecting whether to go with a homeschool Math curriculum (that assumes you need support) or a classroom textbook (that assumes it’s being used in a classroom environment).
There is a difference and it certainly seems to matter in the long run.