Why do fsck_apfs and tmutil report a different number of snapshots?

Question

I'm running macOS Mojave 10.14.6 on a Mac Pro 5,1 and investigating a Time Machine snapshot issue. When I run tmutil listlocalsnapshots /, it shows 20 of them:

Host:~ user$ tmutil listlocalsnapshots /
com.apple.TimeMachine.2023-08-03-110415
com.apple.TimeMachine.2023-08-03-120603
com.apple.TimeMachine.2023-08-03-130847
com.apple.TimeMachine.2023-08-03-150130
com.apple.TimeMachine.2023-08-03-170152
com.apple.TimeMachine.2023-08-03-180816
com.apple.TimeMachine.2023-08-03-190627
com.apple.TimeMachine.2023-08-03-200902
com.apple.TimeMachine.2023-08-03-220218
com.apple.TimeMachine.2023-08-03-230728
com.apple.TimeMachine.2023-08-04-000545
com.apple.TimeMachine.2023-08-04-010829
com.apple.TimeMachine.2023-08-04-020659
com.apple.TimeMachine.2023-08-04-030245
com.apple.TimeMachine.2023-08-04-040702
com.apple.TimeMachine.2023-08-04-050615
com.apple.TimeMachine.2023-08-04-060800
com.apple.TimeMachine.2023-08-04-070944
com.apple.TimeMachine.2023-08-04-080305
com.apple.TimeMachine.2023-08-04-085549

However, when I attempt to repair my disk with fsck_apfs (either booted normally or in Recovery Mode), it fails and reports 22 of them:

Error:
** Checking snapshot 1 of 22
error: btn: invalid key order (2) did 1156985 / oxid 0
    Snapshot is invalid.
** The volume /dev/rdisk2s1 could not be verified completely.

Where is fsck_apfs getting the extra two snapshots? Am I misunderstanding the two tools' concepts of snapshots?

Answer 1

^{This is more of a meta-commentary than an answer, but as owner of many old Mac Pros stuck on Mojave, I feel the OP needs to be aware.}

Do you have a very current 'real' TM backup? If so, I'd just thin the snapshots, hard.

tmutil thinlocalsnapshots / 1000000000000 1

or clear the whole lot

for d in $(tmutil listlocalsnapshotdates | grep "-"); do sudo tmutil deletelocalsnapshots $d; done

Mojave on APFS & Time Machine on HFS+ are not the best of friends.
It's an intermediate form, better in recent OSes but very unready for the swap to APFS at the time. It will, of course, never be fixed, so you'll just have to get along with it the best you can… but don't trust it.

Keep Carbon Copy Cloner clones of your boot drive periodically & use Time Machine as an "Oops, I needed that file" not a total recall of your OS in case of disaster.
See The volume is the wrong format for a backup. Why? for a tale of woe that to this day is still not reliable.

They're often not even reliable for Migration - better to do Mac > Mac over Ethernet, or again a direct clone if the new Mac is capable of just having a new drive swapped in, like the Mac Pros.
See: Migration Assistant fail

DiskWarrior can fix HFS+ Time Machine drives better than anything else, ever - but it can't deal with APFS at all.