iCloud drive seems to have a rather odd bug (especially when we're paying for space!). It doesn't seem to be able to add up the amount of storage correctly, and thus documents take up more space on iCloud than they actually use. For example, my documents folder is ~3.5GB, and in total, on my computer, the entire iCloud drive uses 3.53GB. Yet, in system preferences, this takes 4.2GB away from my available space. So, according to my maths, if we have a 20GB allowance, we can only use 16.66GB of it. I feel a little cheated.