It is dynamically calibrating as you run. It is not mapping. Also I have well over a 100 runs all with my iPhone, except for indoor. I have experienced accuracy from spot on to well over 10% off.If you bring your phone it uses gps. If I run with my phone on my precisely measured 2.0 mile run it registers 2.0 miles every time. When I leave the phone at home it measures 2.1 to 2.25.
I also use a Garmin 620 and Nike+ on my iPhone on every run (3 recordings) and have supplemented with various other devices too.