I should also add that the reason IQ scores are often higher than the index and subtest scores that contribute to them actually does not have to do with ceiling effects. There is a nice explanation (more math than most will want, but enough for the eager math consumer) of the composite effect here, with reference specifically to the WJIV, but generally applicable to most cognitive instruments that combine subtests into cluster or composite scores:
http://www.nelson.com/assessment/pdf/wjiv_asb_7.pdf
