You might find it's got a custom interface board with only a USB port on it, rather than having a bridgeboard to convert from SATA to USB/Firewire/whatever. The 2.5in variety have certainly gone that way...
I'm not sure whats different in those pics, the indent in the top right is a different shape?
I have no idea which I have though, can I check in hdtune?
Found the same thing last week, 2 x Seagate 2TB enclosures were cheaper than 2 bare drives.
The drives in the enclosures were exactly the same models as the bare oem ones.
See my post at:
http://forums.overclockers.co.uk/showthread.php?t=18517119