What type of activity (bike or run)? What device do you have, and what do have others? Do all have GPS devices, or do some of them measure the distance in another way (i.e. steps at running, or wheeel sensors at cycling)?
If all of them use GPS, then the best would be posting a link to your activity, and another one for the same activity from a user with a different pace, so that we can have a look where the problem might be.
