I have not adminstered these tests myself, but I have seen a number of evaluation reports that includes these tests. From my perspective, the WJ almost always yields a higher score than the WIAT. From what test administers have told me, the WJ generally has test items that are shorter and less dense. For my clients (generally students with disabilities), they can score quite well on the WJ because their LD's do not create as much of a drag than they would on more complex, longer test items.
I would ask the person that gave the test to give you an example of test items for each test. I would also ask if any of the tests had time constraints or not.
From my experience - and again I do not deal with a random cross sample of kids, but rather a very specific profile, I see this ALL the time. And I see this same pattern for the writing and reading sub tests too.
