I've thought about this, because I've seen it happen even from grade four to grade five, and I've certainly seen it when it gets to grade seven or eight. I think the problem is that, while number sense is now being taught at the junior level as a whole with several subdivisions, that wholeness does not translate to other strands of math. Teachers in grades six, seven, and eight tend to teach algebra as though it's some strange and wonderful new entity, when it isn't at all; it's a new way of writing what they've been doing all along, and a way of extending their mathematical knowledge to describe change. But if the connections between strands are weak, the connections between parts of a strand will weaken over time, as well.

I also think it has a lot to do with inconsistencies of teaching methodology. I teach using a problem-solving method. I know full well that the grade six teachers at the middle school down the road probably aren't doing the same, or not as completely as I am, because this is my specialty subject and I've been developing my skills in it for two years now; only about one in fifteen teachers has any additional certification in math. I know for a fact that the grade four teacher isn't using a complete problem-solving methodology, because I teach in an open-concept school and I can hear his lessons. Which leaves me the only teacher in three years who is using a consistent, cross-strand, cross-curricular approach to mathematics. Of course their skills will be weak when they aren't consistently reinforced.