If that was the answer she gave you, then I'm guessing she didn't know what she was talking about, but since she was customer support, she at least tried to snow you a little
I am betting the difference comes from a newer, and slightly different tube nozzle design that is slightly smaller at the tip. That would easily explain the slightly lower cfm's and high air fps.
Ask your dealer to let you run the 600, and 770 SBS and see for yourself if you can see any difference between them. The Stihl and Echo are actually so close in numbers that I seriously doubt there's any real detectable (as opposed to perceived) difference. I am betting that the differance that some report seeing is more in their minds than real because they need to believe that the choice they made was the right one. The real difference between the two machines is close enough that any variance in tuning could account for that many cfm's one way or the other etc.