Rebirth trailing behind XVI is really the most surprising statistic considering the PS5 has sold an additional 1 million units since XVI's release. It could be fans of the series don't like separating a game into 3 parts like FFXIII, or the fact that the PS5 is so expensive in Japan, or FF in general just doesn't have the same pull as it once did.
I wouldn't lean into "the game may have sold more digitally" either because Japan is still a primarily physical market and I doubt digital sales increased by THAT much just 9 months after XVI's release.
Still, I'm more interested in worldwide sales. We may get sales figures from SE later today.
You called down the thunder, now reap the whirlwind