I’m not an expert on btrfs, but I assume the inconsistencies come from deduplication, metadata, and maybe compression. I think some of them just count raw block storage, and some include the cost of metadata.

Traditional du assumes that each file takes up it’s full space on disk which isn’t always the case on btrfs. When using btrfs backed oci images, storage can easily appear multiple times higher.

I use btrfs filesystem usage /. I’m not sure that it is the “correct” way, but it works fairly well.