Three dot notion used inconsitent?

Hi,

I just stumbled across the this:

git diff branchA...branchB
--> gives me the diff between (the common ancestor of A and B) and B. That means I never see changes on branchA.

git log branchA...branchB
--> gives me the commits reachable from A and B. That includes changes from branchA.

Is this because of a design decision that I do not (yet) understand or is this inconsistent for historical reasons?

Thanks,
Lars