Of course, what does literate mean? Being literate in one century might mean signing your name, but reading was not a necessary skill. In another, it might mean a 4th grade education, now it might mean a college degree. This article makes for interesting reading.
principals.org/portals/0/content/48743.pdf
I would suggest that the high literacy rates prior to the 1900’s, in addition to not being what we might consider as literate today, did not include all people in the counting. There were slaves, Chinese laborers, migrant workers from Canada, Russians and Spanish in the West, Native Americans, etc. (edited – forgot to include women in the list of not counted)
And perhaps that is why the Pope has some criticisms of modern day capitalism. There is a tendency to say that everything should be done for private money and everyone has a choice and everyone has an equal negotiating position.