You’re making the classic mistake, in that you’re equating standings with sales. They are not the same thing.
So, for example, you can actually sell more copies in a given month, but see your standings go down if other books happen to increase their sales by even more copies. And vice-versa. This is why standings are only the broadest possible yardstick for how well or how poorly a title is doing.
The only number that counts is the number of copies that you sell.
And in a world where Batman, Spider-Man and the X-Men all have multiple titles, and other series such as AVENGERS and GUARDIANS and STAR WARS and JUSTICE LEAGUE and WALKING DEAD and etc, etc have proven popular, it’s going to get crowded up there in the Top-5. You can’t fit more than 5 releases into it in any given month, you know.