... normal office computers, not running data-centric applications, access just 9.58GB of unique data per day.
Round up to 10GB. So in 2 weeks (10 working days) that's an additional 100GB stored locally.
In 20 weeks you've filled up a 1TB drive.
What kind of office (aside from video production) works like that? The ones I know of, most of the machines are used to check email, do data entry on one or two database apps, surf, maybe create some documents or spreadsheets which are then stored on the file server. Other than the database apps, that's less than a couple of megabytes per person per day. And other than temp files, NONE of it should be stored on the local machine.
And if your average user is caching 10GB of temp files then you have a problem with your apps.
I rather suspect that "data" includes stuff like the binaries that make up the operating system and programs that are being used, and simply isn't just documents, temp files etc. that those programs are working with.