There are a number of factors that we look at. For example, direct transportation links between Europe and Canada are much more numerous than between Canada and Africa. At the same time, most European countries are visa-exempt. That makes it easier for individuals from certain countries to access Canada by either flying directly or transiting a third country in Europe, but also by using fraudulent documents of countries that are visa-exempt for Canada.
On April 30th, 2012. See this statement in context.