This makes sense. When you use a copy-on-write block device, it is doing things below the level of the filesystem, so you have to use cow-aware tools to get an accurate view of your used disk space. For example, if you have two files that are 100% deduplicated at the cow-block level, they would show up as different inodes on the filesystem and would appear as using twice the space in the filesystem as they do on the block device. Same would go for snapshots and compressed blocks.