We've seen the same thing with our son. A lot of it has had to do with his verbal communication skills (not great) versus his actual ability to comprehend (excellent). The school has him somewhere around 5th grade reading level (he's finishing 3rd grade now) but testing has shown he's actually closer to 7th or 8th grade. It probably doesn't help that if given the choice he'll spend his days reading comic books and Big Nate, lol.

~amy