Xenoblade Chronicles X could have gotten a downgrade

[captain carot]

Look at the little star. ;)

Unified just means that devs can handle how they share it between GPU and CPU. And that in blocks of a typical size. Now lets say Mario Kart 8 does not need much main RAM. That could likely mean that they can use up to 768MB for graphics.
Though the likely Wii U GPU, i'd still guess on a Redwod based one, wouldn't benefit much from more than 512MB. If it is RV730 based, that did not benefit from 1GB at all as PC graphicscard.

On the opposite we have a Xenoblade X. That might need up to 768MB for data. So there would be 256MB left for graphics.

Now Wii U has unified,but not heterogenous memory. So there's always a rate at which memory is shared between CPU and GPU. The exact rate depends on needs and adressable block sizes. Most of Nintendos typical first party games probably don't need that much main RAM. So there should be more than enough free GPU memory.
Xenoblade might be the first game where Wii U is somewhat short of memory and the GPU might get less than it could utilize.
Having 1.5GB free memory could probably reduce those issues.

That is not more than a smart guess. But it's not unrealistic.