I was just checking the hardware specs on ifixit. Looks like the iPhone4 has 3 microphones in it. So I am guessing they are sending audio from more than 1 mic. I found no such hardware info on the teardown of 3GS and below. Maybe I didn't look hard enough but there does seem to be a hardware-related reason for doing this.

EDIT : I believe > 1 src is needed for fighting noise so you sample the same audio from 2 locations and can distinguish between multiple audio sources in the input signal. This seemed to be one of the selling points of the mic array in the kinect