I don't really know much about Terra Nova, other than it is a group achievement test.
Is this it?: http://www.ctb.com/ctb.com/control/ctbProductViewAction?p=products&productId=745

Were social studies and science portions also given?

Achievement test scores are highly correlated to IQ scores, but they aren't perfect by any means. How gifted/bright did you expect?

99% in Math is very high. Some kids are gifted in Math and 'Above Average' in other subjects. But if you aren't seeing scores that reflect what you see at home, full testing, from the school or privately might be a big help.

Best Wishes,
Grinity

Coaching available, at SchoolSuccessSolutions.com