Here's a script showing the predictability of this post-mount extra data that is created. When the previous snapshot contained 1024 files, the extra data written after mounting the received snapshot is proportional to this. Taking a snapshot of this extra data shows ~1024 "OBJECT" and ~1024 "FREE" lines in the zstream dump, with the raw data for the objects all starting with "ZP/"
Side-note: I also note that immediately after creating any dataset, the "written" is non-zero, a similar effect to this post-receive data?
diff_similarity() {