question about snapshot_create onchange

Hey there!

I'm using `snapshot_create onchange` in my `btrbk.conf` and it works well for some of my subvolumes, but not so well for others.

Could you explain to me, why the later one of these two has been created, even though there is no changed files? I don't know what the "snapshot" line that is returned tries to tell me...?
```
# btrfs send --no-data -p /.btrfs_root_hdd/@ssd_backups/@.20210731T225401+0200 /.btrfs_root_hdd/@ssd_backups/@.20210731T225602+0200 | btrfs receive --dump
At subvol /.btrfs_root_hdd/@ssd_backups/@.20210731T225602+0200
snapshot ./@.20210731T225602+0200 uuid=069a7066-d114-7246-8cee-fd2f16a6f056 transid=125353 parent_uuid=de9581cb-3bac-ae47-9752-0e7ca2355b85 parent_transid=125353
```
A few minutes later now, and it tells me access-time, modified-time or creation-time of / and /etc have been changed..
```
# btrfs send --no-data -p /.btrfs_root_hdd/@ssd_backups/@.20210731T225401+0200 /.btrfs_root_hdd/@ssd_backups/@.20210731T231201+0200 | btrfs receive --dump
At subvol /.btrfs_root_hdd/@ssd_backups/@.20210731T231201+0200
snapshot ./@.20210731T231201+0200 uuid=89ea2cd9-72a3-d94d-a9e2-9282dea9eed4 transid=125408 parent_uuid=de9581cb-3bac-ae47-9752-0e7ca2355b85 parent_transid=125353
utimes ./@.20210731T231201+0200/ atime=2021-07-31T22:25:11+0200 mtime=2021-07-31T23:00:02+0200 ctime=2021-07-31T23:00:02+0200
utimes ./@.20210731T231201+0200/etc atime=2021-07-31T22:26:11+0200 mtime=2021-07-31T23:03:30+0200 ctime=2021-07-31T23:04:02+0200
```
Strange, since all the relevant lines of my `/etc/fstab` have `noatime,nodiratime` in the options column.
I don't figure these directories have been recreated, and I believe only files can be modified, not directories?
Or I guess, maybe a file creation / modification within a parent directory counts as a modification for the parent directory? But if that happened, it should show those child items in the btrfs snapshot difference list too... hmm..