There are various methods of calculation the duration of an activity. The most straight forward is Elapsed Time. This is the total time between pressing Start and Stop for your activity. This may include pause time or other time you were not actually working out. This is why we default to Moving Time. This is the time you spent working out. This excludes any (auto) pauses which might have part of your activity. This makes a more fair comparison of activities in general.
The time you will see in your profile and results will typically be your moving time. On the activity details screen you can also see the elapsed time, when it's available.
Only for Track Challenges, where the purpose is to follow an exact predefined course, we take the Elapsed time (when available) as for this type of challenge it is important to include the pause time.