Strange reachability inconsistency (apparently, at least...)

I have a (fsck-clean) git tree in which for 2 commits A and B:

* "git merge-base --is-ancestor A B" returns 0
* "git log B..A" returns a non-empty set of commits

I get this behaviour with 2.3.0 as well as with 2.1.3 and 1.7.12.

Is that a real bug or am I just misinterpreting something ?