That works. Can you explain why? Does diff see this as a single commit or as set of commits? If it sees it as a set, how does it decide what two commits to diff from that set? If it's just one, what does it decide to diff?

I'm kind of confused because gitrevisions(7) says the triple dot is symmetric difference, but exchanging master and experiment does not produce the same output from diff.

Not so helpfully it has a different meaning in 'git diff' than in 'git log'. Basically, it means the difference in the second branch from the first common ancestor of the two branches.