The first reference quite explicitly says "number of copies each book sold over the last 50 years."
Sales is an approximation for readership. The second source also claims "figures may incorporate populations of secular/nominal adherents".
So, while I agree that populations that identify with Christianity may be larger, it's pretty obvious a lot of those do not observe the rituals the religions they identify with specify.
To get a better readership proxy, perhaps, one could use the share in sales of religious texts across the different demographics.
Then we need to also define what "read" means. Does it mean the book was read cover to cover or does it mean the book is read from occasionally? Religious texts are used in very different ways than other forms of literature, to the point comparison borders the questionable.