The majority of children need that repetition to even recall how to do basic addition, subtraction. Do you know how many children struggle with basic arithmetic all through elementary school. In my school district at least, there was a tiered system that seemed to work very well. You were in an essentially randomized teacher's classroom in elementary school (out of 3 classes per grade). Then you were split into high, medium, and low groups, and actually switched teachers for math section, even in elementary school. Within each of these groups, there were 3-4 subtiers each with 5-8 students, except for the highest of the high, where they pretty much just sit you down with an algebra book and tell you to go to town.

As long as teachers make this sort of differentiation among students, they are all getting (in the teacher's judgement at least) the exact subject matter / practice time that they need.

I don't think your suggestion that only some students see a fraction by grade 6 is necessarily valid. There were 8 students in my middle school class of about 300 that had a teacher shipped in from the high school to teach Algebra 1 in 6th grade, whereas there were other students that had a specialized two-year Freshman-Sophomore Algrebra 1 curriculum.

I don't know when / where you were in school, but at least in Missouri (a region not exactly known for pushing education bounds), differentiation is pretty common, in math / reading. Science / history are another subject (pardon the pun).