IQ

Many place too much weight on the value of an IQ.
The truth is, one's IQ is not a definitive indicator of one's future success.

Think of and IQ test as one would view the results of an NFL combine.
Sure, one may test well on what has been determined to be desirable traits and skills but they may still be a flop when put into game situations while competing against equally talented players.

Think of Tom Brady.
His combine performance was so bad that the guy was taken with the 199th pick in the sixth round of the 2000 NFL draft.
Six quarterbacks were taken before him.
(The 199th pick? That's like getting taken after the girls and the handicapped kids at a schoolyard kickball game.)

Brady initially struggled in high school, college and in the NFL - often only gaining the starting role after the first string quarterback went down with an injury.
Brady needed a mentor to help him with his confidence at Michigan where he even considered transferring to California.

For an overachieving scrub he seems to have benefited from some coddling.
Luck also seems to have played a part in his success.
Being placed into systems which took advantage of his skillset also seems to be a factor.
Most importantly his drive and hard work were used to his advantage because he had some help along the way.