confused about a conflict during octopus merging

Dear list,

I am a little confused why the following yields a conflict:

$ git init
$ touch a; git add a; git commit -ma
Created initial commit 0ba90cc: a
 0 files changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 a

  # create merger branch for later
$ git branch merger

  # create a file
$ date > d; git add d; git commit -md1
Created commit 89d22eb: d
 1 files changed, 1 insertions(+), 0 deletions(-)
 create mode 100644 d

  # branch off a second branch and populate it
$ git checkout -b second
Switched to a new branch "second"
$ touch b; git add b; git commit -mb
Created commit 7370737: b
 0 files changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 b

  # modify file changed in previous commit
$ git checkout master
Switched to branch "master"
$ date >> d; git add d; git commit -md2
Created commit bb5f1b4: d2
 1 files changed, 1 insertions(+), 1 deletions(-)

Now the tree structure is

  x -- master: d2
  | x -- second: b
  |/
  x d1
  |
  x -- merger: a

  # now octopus-merge master+second into merger
$ git checkout merger
Switched to branch "merger"

$ git merge master second
Trying simple merge with bb5f1b4f3a8cbfcbc9dbc97cc823f3ca05db2db1
Trying simple merge with 7370737ff3aa6540a425db015e194766d2cbda2e
Simple merge did not work, trying automatic merge.
Added d in both, but differently.
error: Could not read .merge_file_mCwa4W
ERROR: Merge conflict in d
fatal: merge program failed
Automatic merge failed; fix conflicts and then commit the result.

I don't understand why these conflict. I mean, sure, it makes sense: after merging master, file d is at d2. Now when merging second, it tries to replace d by its ancestor, d1.

What I don't understand is why Git does not understand that commit d is part of the path of arriving at d2, so when 'second' is merged, Git *should* really see that the change to file d in 'second' (d1) is an ancestor of the already present d2 commit, and *not* conflict.

What am I misunderstanding here?

Thanks,