The scale is measured where the pan starts and stays fixed for the whole gesture, so the speed never changes while the finger is down. The default.
The scale is measured at the centre of the screen, so it depends neither on where the finger started nor on where it goes - every pan moves the map at the same rate.
The map point under the finger follows it exactly. On a tilted view the speed then changes during the gesture, accelerating as the finger moves toward the far part of the screen.