HOW DOES THIS EVEN WORK?

I thought this was one of those non-sense problems but I literally can measure the difference between two batteries like this just by hooking up positive to positive

I just don't get it, theres nothing for my meter to consider ground? batteries must act different when they're arranged like this because if I isolate the system, and say measure the positive of one batter and the negative of another, no voltage even though my meter is definitely seeing +9v on one end and -9v on the other