i get about 20 hours on my RvU, and 20 on my v8
think of it like driving a car. If you floor it, shift often, change lanes, ect... your going to get worse milage than just driving in the slow lane on cruise.
Maybe you typically do more on your 2.1 than you do on you 2.2, i know i barely use my 2.2 at all, and my 2.1 has been dramatically toned down. But people who are constantly playing with their 2.2 or 2.1 will have different performance. And this all in addition to the drastic software changes between versions. Just keep it in mind