NOR is a type of flash and these NOR images are designed to directly fit on NOR flash memory, that's why they are called NOR. And the BRN images are designed to be flashed by the brnbootloader and contain the appropriate headers. the VGV7519 had a brnbootloader by default and can be accesed either through the TTL connection or by holding the reset button while powerup the VGV7519. Unfortunatly the brnboot is password protected and this password is not yet known. So the images should work but since the password is unknown there is no way to test them. They are included/build because the code to make them was already present. Because there is no way to flash a image through the brnbootloader it has to be replaced by uboot as a workaround.
This is also why there is only a BRN factory images, and NOR sysupgrade images. The difference is the header information for the factory flashing procedure.