This resource explains why “math” doesn’t make sense with ZFS snapshots.
Box with white sticker = block in the live filesystem
Box with colored sticker = block protected by a specific snapshot
Translucent box = block that still exists in the pool (protected by a snapshot) and still consumes space, but it is no longer part of the live filesystem
I didn’t have time to expand on the resource, but I might in the future. One thing it doesn’t show is how it relates to replication.