Wireless update is different in that it checks what's different between your current OS and the new OS and only downloads and updates those files. That's why it's smaller then a full blown OS you'd download to your PC.
The carrier's have to actually release the OS for you to download wirelessly, and they don't have them all out there....yet. I've found the wireless update to be about 1 update behind what you can download from the web.
No longer a BES Admin, but it was fun while it lasted!