I'm not 100% sure why Apple would do such a thing. But I have my theories. First some more background on the format.
Apple's posted a quick KB article on the format. From the KB we know that the resolution is hard set at 960x540. That is exactly a quarter of 1080P HD size. It is also 20 pixels away from being four times the size of the iPhone screen. A nice round multiple for easy up and down sampling?
The iFrame Video format is designed by Apple to speed up importing and editing by keeping the content in its native recorded format while editing. Based on industry standard technologies such as H.264 and AAC audio, iFrame produces small file sizes and simplifies the process of working with Video recorded with your camera.
So why does Apple go and create a whole new resolution stantard when 720P and 480P are already very popular? Apple not only created it but they've built the iFrame brand around the new standard with the logo above. This isn't just some random resolution.
Is 960x540 a good resolution for a Tablet? Only a small, six-inch one in my opinion. It is also a good size for videoconfrencing over "almost-HD" iChat. Perhaps this size allows for streaming over "ordinary" pipes.
Its size, mentioned above, means that it will easily scale between iPhone size and Full HD. Maybe that's just it. Apple wanted a default size for consumer video taking that would upand down scale easily an efficiently.
We'll find out soon what Apple has in store for us.