One can be "intellectually" gifted while not academically gifted. Also, sometimes a child scores high on an IQ test but their lowest score is working memory. Working memory is related to doing well academically. I have been reading a lot recently and I read somewhere that in elementary school a lot of their learning relies on working memory--so you could have a gifted child with average working memory which would explain the grades. Or maybe just like you say...she is only going to perform if she is interested.
