FSIQ includes things like working memory and processing speed as well as reasoning ability. Even when reasoning ability is very high, low scores in some areas not highly correlated with reasoning ability, such as processing speed, can lower the FSIQ (which, strictly speaking, shouldn't even be calculated if there are significant spreads between scores). So it is possible to have very high reasoning ability, indisputably in the gifted range, and still have an average FSIQ. The score profile will have significant spikes and valleys, rather than having scores clustered in a relatively narrow range.
