Today my grade 3 daughter did grade 2 math, grade 3/4 grammar, grade 6 geography and something approaching grade 8 or 9 science.
Since we started a more structured approach those little numbers after grades have concerned me a little more but I think today I got a good look at how useless they can be at times. It's not the numbers, it's the child's skills and interests.
It's funny, I knew this when I was a radical unschooler. We took a different approach and now I seem to have to learn it all over again in a different context.