The implication of this hypothesis, and research into the subject in general, shows that test outcomes do reflect at least in part cultural factors. There are "cultural differences in valued and therefore trained strategies to solve certain cognitive tasks" as well (Ibid). That these differences have been identified within cognitive science illustrates that cultural bias does not simply reflect differences in cognitive potential among members of certain groups, but rather that it reflects differences in the ways that cognitive potential is operationalized. Shiraev and Levy (n.d.) argue this case further. They cite research that shows that people adapt the way that they operationalize their intelligence to their local setting. They cite the examples of an Indian chess master, who uses the same psychological mechanisms in playing chess as a farmer would use to secure a deal on a new tractor. The example is apt -- chess-like problem solving strategies are found on IQ tests while bargaining is not usually found on some tests.

In contrast to the many studies that have identified clearly the difference between cognitive...

Where differences have been identified, there is seemingly a number of causal factors that contribute to the inability of that group to realize its potential, including familiarity with testing, the degree to which the test reflects that group's cultural norms and the way that group applies problem solving. There remain individual differences in cognitive potential that are partially reflected in IQ tests, but there does not appear to be any evidence to support the idea that there are innate differences in cognitive skills between different groups.
Works Cited:

Malda, M.; van de Vijver, F. & Temane, Q. (2010). Rugby vs. soccer in South Africa: Content familiarity contributes to cross-cultural differences in cognitive test scores. Intelligence. Vol. 38 (6) 582-595.

Shiraev, E. & Levy, D. (no date). Cross-cultural psychology: Critical thinking and contemporary applications, fourth edition.