Because you’re talking about different numbers that indicate different things. If you’re talking about, say, the Top 10, that’s a metric as to which comics sold the most–but not how many copies they sold. That information is not disclosed, never disclosed. As I delight in pointing out to people, it is entirely possible for a book to fall in the rankings even if it sold more copies–because you are measuring different things. The reverse is also true. Market Share is a real number, Ranking is a real number, Sales is not.