Why is it different? Well, because the OVF is more than the virtual machine format. A virtual machine format like for example VMware's VMDK or Microsoft's / Citrix' VHD basically is only about the virtual disk. The OVF describes the entire virtual machine: this includes the full list of required virtual disks plus the required virtual hardware configuration, including CPU, memory, networking, and storage.

This OVF is seemingly very cross platform from what I can tell this is a join effort from VMware, Microsoft, XenSource (Citrix), IBM, HP and Dell. Cool stuff!

Read LOTS more about the Open Virtual Machine Format and learn create "OVFs" (with a tool) for yourself.