Windows 10 does a lot of weird stuff, UEFI I think it is in particular. I only have a tiny bit of experience with it, but there is a separate partition on your hard drive where a lot of core stuff is now stored and organized. not sure how much it does or doesn't actually contribute to what you're noticing.
I also notice on my Windows 7 machine that there is a Logs folder that is like 10GB. the logs folder apparently grows without bound over time. might have been fixed in later windows versions.
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