"The subscriber count is more volatile than it used to be because players have gotten older and don't have as much free time anymore. As long as players are happy and enjoy the game, the team doesn't mind extreme peaks and valleys. They just want to ensure you have a great experience and feel like you got your money's worth.", Blizzard 2015.
I am guessing that FFXIV's numbers are going up because players there get younger and have much more free time. Makes perfect sense now.