Bear in mind that many countries did not take part in PISA, such as India, which dropped out after a trial run in two states produced average scores below any seen on this chart. For a broader sampling of Third World scores, see the 2011 TIMSS Math and Science scores.
The reality is that there is not much difference in PISA or TIMSS scores within major racial blocs of countries. The Northeast Asians all tend to score well, the European and white Anglosphere countries tend to score fairly well, the Latin American countries tend to score fair to middling, and on down from there. The rank order of continents is very much like the rank order of racial/ethnic groups on NAEP or SAT or CST tests. Newcomers to the topic like Amanda Ripley, author of The Smartest Kids in the World, get excited about minor differences in PISA scores within continents, but those often are statistical noise.