Many participants run with tracking devices such as a watch or an app on their phone. These devices are great for recording runs and giving an indication of pace, etc.
However it not unusual for your device to record a distance with might be shorter or longer than the official race distance. Tracking devices receive their location from satellites and can be less accurate in certain situations where there are tall buildings, tree cover and large groups of people.
Additionally our race routes are officially measured and certified by qualified course measurers who accurately measure the course along the “running line” which is defined by international standards. Generally speaking it is the shortest line whilst staying on the road and a constant distance from kerbs and the like. When running with many others during the event it’s not always possible to stick to this “running line”.