> The result was that, despite lacking 6 years of formal math education, the students in that district were only one year behind the other students when they went to junior high.

If it is anything like German math education back when I was in school a lot of the early things are simplified to an insane extreme and either have to be revisited later or are completely replaced with a better approach. I think we went through half a dozen ways on how to multiply and divide numbers when only one mattered in the end. If you learned the basics from your parents you could even expect to be penalized since you had to use the useless methods described in the curriculum for the first few years and could get penalized for correctly identifying an equation with a negative result.

> Basically, it seems like kid’s brains are just not able to efficiently learn math

Or it may be related to a really bad curriculum that tries to be "age appropriate" by teaching mostly useless crap.