Why do updates refuse to download when your free space gets close to being full?

I had 40 gigs left on my hard drive. I still have the original model.

Battlefield 1 had an update and even though i had the space for the download, it wouldn't do it. This has happened with multiple games so i had no choice but to delete some of my smaller games.

Why does this happen? Why do i need to delete stuff off of my system even though i have the space for it?