not their IQ, their testing abilities. I do really really well at standardized tests because I have taken crap of them and I have been coached. my IQ may not go up or down, but my tests scores will fluctuate. if I retake that test and always choose the same answer location (top left for instance) my score test would plummet. In your world I would then be "retarded". except that it wouldn't, means I did crappy on a test.
people do different things better than others. test taking is one of those.
even looking at standardized tests will show you that you can be smart and still do bad in certain areas. I will use myself as an example. My ACT score was a 31 as a composite. seems pretty good. you break it down my scores were something like 36 in math, 34 in science, 33 Reading, 15 English. I am/was top 5% in most of those categories, then that last one I am barely a Neanderthal.
what does it mean for me as an individual that my English score was low? Not a dang thing except for how well I can type/write. This test was just patterns.
a test can only tell you as much as is in it. that one was certainly no end all be all.