help understanding git diff output

Hi,

Could you please  help understanding the output of git diff.

I have two branches with this type or relationship:

0-a-b-c-d-e-f-g (branch1)
\a1-a2-a3-b-c-d1-d2-e-f1-f2-g (branch2)

Where commits a1,a2 and a3 as well as other numbered commits in
branch2 are the split version of their counterparts in branch1.
The the branch2 was created by creating the branch2 from branch one
and rebased on 0 with few commits being cplit up during the rebase.

I.e. the resulting state is the same for both branches.

I get empty diff when i execute:

$git diff branch1..branch2

and i also get empty diff when running

$git diff branch2..branch1

What i cannot wrap my mind around is why the command below with
symmetric difference range gives me non-empty diff

$git diff branch1...branch2

Please, advise

Thanks a lot,
Eugene