Take the sales with a grain of salt. Apparently their metrics are inconsistent at best. (Eg. Counting overall users, counting remasters for some but not others, whatever they did to make FFIV TAY higher than FFIV).
I caught this on Reddit last week or something, there was lots of discussion about it there, how various different things were throwing the scales off. Shame it wasn’t accurate or clear, it’s a good idea.
I'm honestly surprised the number is as low as it is though. In its prime FFXI did have a lot of players, even if it's only in maintenance mode these days (although I am stoked for the upcoming FFXIV alliance raid series based on FFXI!)