Probably “leftovers” from apps, and temp files.
A restore does not copy your apps (and temp data) themselves back, but it just downloads the latest app versions of your apps.
Yes, it may be that this is the reasoning. Nevertheless, the amount of storage freed up is more than significant for me.
Unfortunately, I don’t have a recording before the restore for comparison. But now there is actually at least 1/3 more space available - more, in fact.
It sounds a bit like voodoo, but if someone’s storage is running low, they could try restoring a backup to free up more space. If a backup has never been restored or has not been restored for a while.