You may have uploaded a GPX file to FATMAP and see some unusual extra lines on the track, such as the example below.
This occurs when your tracker has a hard time getting triangulated by GPS satellites. This can happen in deep valleys, heavy tree-cover, or any area where wide-open skies are more rare. These bad data points throw off the rest of the track in all respects - from statistics like average speed, total distance, and the elevation profile to visual inconsistences like shown above.
You can help prevent this by ensuring your device has the best possible signal when tracking, mounted high up and clear of obstacles. Give your device time to locate an accurate signal before you begin moving, and try to avoid starting the tracker when you are inside a building.
Within FATMAP you can edit your route and remove these bad data points: How can I edit or delete my routes?