Confusion about diffing branches

So I imagine I'm missing something, or perhaps I'm just looking at this from the wrong perspective, but here is what I'm seeing. Lets say I have something like:

         A---B topic
        /
   C---D---E master

If I want to see a diff of all of the changes between the two branches I can say:

   git diff master topic

Which shows me the combined diff of commits A, B and E. That is exactly what I would expect however, I would expect that equivalently I could say:

   git diff master...topic

to see all commits reachable from topic and master but not both. However, this doesn't do what I expect but instead only shows me the combined diff of A and B. Likewise:

   git diff topic...master

Shows me the diff of E. Am I crazy or isn't this supposed to be the behavior of the topic..master notation? Strangely enough running either

   git diff master..topic
   git diff topic..master

both show me the diff of A, B and E, which is what I would expect from the master...topic notation. Am I the only one who thinks this is backwards? The same experiment using git log shows me what I would expect so somehow I think I'm missing something.