GOOGLE IMAGES: GM Testing VOLT will get you to a photo of dust intrusion tests conducted on the VOLT at a US Army facility.You can see the clean separation line at the boot/trunk.
Can you provide a link to the particular image you reference, here? I looked and there quite a few images listed; I didn't really see a "clear separation line" (or didn't know what I was looking at) in any of them.
Judging by that dust test, GM could have made the Volt bigger, and stretched the rear a bit to make the rear deck lower to a lower Cd, more space for batteries, thus higher range, and just a bigger car to cater to Americans better.