I wonder if it's basically anything that goes through that phone to be uploaded by Google Photos just doesn't get counted in your storage. So like, even if you have the Pixel, and another device, only photos uploaded from the Pixel are part of it. And if that is the case I presume it just has to be until they change how all their storage works.
It's only photos and videos from the Pixel device. Make sense that the storage is probably "forever" especially with people here changing their phones like every 3 months.