zpool geli encryption question

A machine periodically backs up bhyve volume-backed VMs like so:

# zfs send ssdzfs/fbsd140R | gzip -c > /vol-backups/$(date '+%G.%m.%d_%H:%M').fbsd140R.gz

This vm is zfs internally with geli encryption of both the fs and swap.

The same backup routine applies to an openbsd vm. It has its own way of
filesystem encryption.

Both volumes are 64GB in size. On the host, both volumes use lz4.

Surprisingly (to me at least), the freebsd backup results in a smaller
size of archive. The openbsd one results in a slightly larger archive than 
its source.

I'm expecting both archives to be slightly larger than their sources,
because encrypted data is uncompressible.

The freebsd archive is 19GB. The openbsd one is 65GB. Why is this?

--