Many skills impact understanding oral directions, with auditory processing, attention, and receptive language definitely at the top of the list. In what way do you think these do not fit? Not everyone presents the same way, so just because he doesn't look like the family members with ADHD diagnoses does not mean the diagnosis doesn't fit him. He is definitely on the young side for an auditory processing evaluation, but not too young to see early symptoms of ADHD.
Another possibility that affects understanding directions, and doesn't necessarily have to mean ADHD or APD per se, is (working) memory deficits/differences.
BTW, remember that Terra Nova is an achievement test, not a cognitive test, so it is not all that strange that it may not always line up with cognitive testing. Also, in a child for whom attention may be questionable (which actually describes pretty much all six-year-olds!), group testing is often less reliable than individual testing. Did they give the online or paper version? That can be a factor as well, since some children are not as comfortable testing on the computer, or are distracted by the format.
