I've seen that type of thing from a lot of unschooling and homeschooling parents (not here, on other forums where I've seen a lot of those families posting), and I've never gotten that either.
There's a lot of 'we take our kids to the museum!' 'we took our kids for a walk and discussed why leaves change colour and collected leaves!' 'we got our little kid a science kid and do experiments!' 'our X year old already knows about Y!' as if that stuff doesn't happen in other families. My parents sent me to school, and yet did every one of those things with me, regularly.
I used to be given sentences to correct as little kid busywork; I had a whole leaf collection kit; I had numerous science kits; I had my own little microscope and terrorized everyone around me taking samples of various cells - most of this stuff was done looong before it was ever addressed in school. In my world, my parents were not at all unusual. I know there are many parents who aren't involved and etc., but plenty, especially those who value education, are. That behaviour is normal to many people, hence science kits and all exist to be sold to kids.
I don't know where the idea comes from that parents who send their kids to school don't do that on top of schooling or that it somehow substitutes for schooling. We did entirely other experiments in science class in school - and had to work together and discuss the different possible outcomes and things that would cause them and etc., etc., etc. that I didn't have to do at home.
Like I said, I know there are parents who don't know, don't have the time and/or funds or etc., but plenty of parents do as a matter of course AND send their kids to school.