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.

* Click on picture above to open tab and make picture large and legible

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.