Apple always lowballs guidance (note Apple beat its own guidance by about 2B I believe) so its up to the street to set their own view of what Apple should be making (on top of Apple's estimations). Of course they're just guessing based on past results so its way less accurate.

Its a bit of Apple's fault however. They're noted for low balling revenue guidance which is why the street likes to tack a few billion on whatever Apple says.