Strange behaviour of git diff branch1 ... branch2

I came across this odd question on stackoverflow:
	http://stackoverflow.com/q/13092854/1507392

If git diff is run with "..." as a separate argument between two commit-ish arguments causes it to produce strange output. The differences seem to be the same as if "..." was left out, but change lines begin with 4 + or - characters rather than just 1.

Can anybody explain what is happening here? I don't have any reason to want to use that form myself, but I'm very curious about why it produces this odd output.

Thanks.