Avery Pennarun [off-list ref] writes:
So what git tries to do is reduce the diff output of a merge commit
down to just the "unexpected" parts: the parts that are different from
what you'd get if you just merged in the individual changes in the
obvious way.
This (and everything you said after this paragraph) is not quite correct.
What is shown is the pieces that do not match either of the parent.
So if you have a conflicted merge and resolve it by taking what one side
did literally (think: "merge -X ours"), that hunk becomes uninteresting
because the end result matches what one of the parents had.