The most likely cause of the discrepancy is that your stride length is set different from your actual stride length.
That is, if you set the stride length to be your walking stride length but you are actually running the distance, then you need to set the pedometer to your running stride length. Your running stride length is most likely much longer than your walking stride length.
There are at least two ways to determine what your running stride length is:
1. Have someone watch you run and mark off ten running strides in the middle of your run. So you would run 20 steps and your partner would mark off the 5th and 14th step. Then measure the distance and divide by 10.
2. Change the stride length repeatedly until the distance equals the measured distance.
The pedometer does not use your stride length to count steps. It uses it only for calculating distance.
If you changed the stride length right now, your recorded distance would change right now but your step count would remain the same.
If you are using the pedometer to record both walking steps and running steps, you will not be able to get a correct distance but you will get a correct step count.