In the interest of realism, I've been studying the numbers as best I can.
Amazon claims to have over 600,000 titles for the Kindle. However, we all know a good chunk of these are public domain works they've snatched up and people who are using the Kindle to self-publish for free. There are no hard numbers, but I'm going to use the 50% rule and say that this accounts for maybe 300,000 Kindle titles. This leaves a touch over 300,000 paid titles, which is what the ranking goes off. And in that case Ex-Heroes is (currentley) #2181 in paid titles.
So, still solidly in the top 1%, but not quite as spectacular as the #1 slot makes it sound.