It really depends how you look at it. It doesn't means that #20 is more successful than #5 for example, or that #10 is more successful than #16.
#20: 10 million albums, 2 million singles
#5: 5 million albums, 20 million singles
#10: 1 million albums, 1 million singles
You see? The ranking isn't about who is more successful. It's just about the balance between the success (even if it is moderate success) of their albums and singles.