ORIG_HEAD after rebase is confusing

Reading the git rebase manual and some answer on stackoverflow I assumed `ORIG_HEAD` will point to the original HEAD, the tip of the branch *before* I started rebasing. But it doesn't seem so.

For example, I have this:

  $ git log --graph --all --oneline
  * 9830f9f (master) X
  | * fb7b6a6 (HEAD -> feature) D
  | * 46b7a7a C
  | * da5e4c7 B
  | * 5c135da A
  |/
  * 6848823 Init

  $ git rebase master
  Successfully rebased and updated refs/heads/feature.

  $ git rev-parse ORIG_HEAD
  da5e4c7e9eb3b10c1efa08c534b9c9e4b92d9fd7

  $ git reflog
  a647bd7 (HEAD -> feature) HEAD@{0}: rebase (finish): returning to
refs/heads/feature
  a647bd7 (HEAD -> feature) HEAD@{1}: rebase (pick): D
  2f458e8 HEAD@{2}: rebase (pick): C
  0aa2160 HEAD@{3}: rebase (pick): B
  b957fc7 HEAD@{4}: rebase (pick): A
  9830f9f (master) HEAD@{5}: rebase (start): checkout master
  fb7b6a6 HEAD@{6}: checkout: moving from master to feature
  9830f9f (master) HEAD@{7}: commit: X
  6848823 HEAD@{8}: checkout: moving from feature to master
  fb7b6a6 HEAD@{9}: commit: D
  46b7a7a HEAD@{10}: commit: C
  da5e4c7 HEAD@{11}: commit: B
  5c135da HEAD@{12}: commit: A
  6848823 HEAD@{13}: checkout: moving from master to feature
  6848823 HEAD@{14}: commit (initial): Init

So `ORIG_HEAD` here points to the original B commit. (I expected the D.) Honestly, this doesn't make much sense to me in that I don't know *why* it even chooses B which is a middle commit in the chain. (And from reading the source `sequencer.c` I can't deduce it either.)

  $ git --version
  git version 2.29.0.windows.1

What I actually wanted to do was `git reset --hard ORIG_HEAD` fwiw. And for example `git diff HEAD..ORIG_HEAD` to check for unwanted changes after a merge conflict.

Regards, Caspar Duregger