This article here might help your understanding this better (especially the concept of allocation & the difference between used space and allocated space): http://www.infotinks.com/btrfs-fi-df-balance-btrfs-df/
I didnt have time to make it fancy shmancy but this should get the point across. JUST CLICK ON THE PICTURE, IF YOU CANT READ IT, TO MAKE IT LARGER.
This masterpiece needs to be updated to include “btrfs fi usage”, but I think the output of it will be pretty self explanatory. Read about it in the link above. Here is a link to how the output of “brtfs fi usage” will look like.