This is a bit confusing to me. From the article "Rather than just use the accelerometer to count steps, the Band combines that data with your phone’s GPS data

This is a bit confusing to me. From the article "Rather than just use the accelerometer to count steps, the Band combines that data with your phone’s GPS data and the always-on heart-rate monitor to figure out the length of your strides, which provides a much more accurate measurement of calories burned." If there is on-board gps, why does the MS band need gps data from the phone ?