Unexpected merge results in git-svn repo

Question

I am getting some unexpected results from a merge and I'd like to understand why.

I have two commits X and Y that I want to merge.

git merge-base X Y # yields B
git diff B X -- F # is empty
git diff B Y -- F # contains the change I want merged
git rev-list X ^B -- F # is empty
git rev-list Y ^B -- F # contains one commit

git checkout X
git merge Y

fails with fixable merge conflicts on other files, but uses X's copy of F instead of Y's. I was expecting it to use Y's copy of F, since only Y has modified F since B. What could cause this?