Well, the reason I'm looking at the numbers is to make sure there isn't an updated model. If they did update the model, I would rather get the new one. I talked to echo customer support on the phone just now, and found out some information.
The person said there has not been an update on the blower, but from what she sees the 756 cfm and 234 mph are accurate. She said it is most likely due to new tests showing different averages, or there was a misprint in the past. I highly doubt there was a misprint that they let go for as long as they have, so it must be new testing. With that being said, maybe that is why a lot of people say the 770 blows better then the br 600 magnum. With a difference 33 mph and the little difference in cfm, there should definately be a slightly noticable difference.
The person may have no idea what she's talking about, but it makes sense.