I think that's a fairly reasonable guess for why the math isn't going up so quickly. But I'm also thinking that the huge growth in reading isn't average - those seem like big jumps to me (though I'm no MAP expert - inky hopefully will jump in!) But I can also see how the reading would jump up higher at your DS's age, because the reading materials at his level are probably getting more interesting and he has probably been exposed to more rules of grammar to increase his scores there. I'm guessing that there was not a whole lot of that in kindy.
My DS had a bigger jump in math after we switched to a school that taught math the next grade up, so he was exposed to more things he wouldn't have been before. Since the MAP really is just testing what you know, if you are not exposed to it, you cannot improve much.
