Your btrfs snapshots are possibly counted separately by all the regular tools. They simply go into every directory they can find and add up the size of the files they see. They do not care if they are looking at an identical snapshot of the folder next to them, they simply add it all up.
Use sudo btrfs filesystem show
(and maybe add a path behind it, I am not sure). That will give you the true usage.
And basically sentenced for life on top of that.