What does Gladwell mean when he states, "The outlier, in the end, is not an outlier at all" in Outliers: The Story of Success?

By this statement, Gladwell means that outliers are not really what we might consider outliers—that is, outstanding individuals who have gotten ahead because of their amazing personal qualities. Instead, they are people who have benefited from what Gladwell calls "parentage and patronage" (page 20). While people who achieve great success may appear to have succeeded because of their personal qualities, they have benefited from years of advantages and opportunities that others do not share and that help them learn and grow.
Therefore, it is important to consider the largest cultural and societal environments in which people develop. Their success is a function of the way in which their culture or society is able or unable to provide them with advantages. For example, a student who seems bright and capable is usually rewarded with more opportunities and attention than a student who is not always able to present his or her best at school (possibly because this student has a more chaotic home life). The student who receives more opportunities will generally go on to achieve greater results; therefore, one's success is also a function of what one receives from the society around him or her.

Gladwell begins the book with the definition of what an outlier is: a person who is seen as “markedly different” from everyone else, either in terms of extraordinary talent, success, or both. Throughout the narrative, Gladwell provides examples of these types of people, like computer entrepreneurs and Canadian hockey players. What he finds in his research, however, is that each one of these individuals had some kind of advantage beyond his or her control. These included historical timing, cultural differences, and even some beneficial nuances of language. It turns out their successes are not only explainable, but sometimes even predictable. This conclusion means outliers are NOT markedly different individuals after all. Gladwell says exactly this on the last page of the text:

They are products of history and community, or opportunity and legacy. Their success is not exceptional or mysterious. It is grounded in a web of advantages and inheritances, some deserved, some not, some earned, some just plain lucky – but all critical to making them who they are. The outlier, in the end, is not an outlier at all.