When workouts contain SMART intervals with variable duration (what are termed target MPA or TPMA intervals), the interval duration will be dynamic and can be longer/shorter than the workout’s default duration based on how you perform it.  For example, if you tend to recover at a higher intensity than the interval target, your workout may end up being longer.  Likewise, if you tend to overshoot the high intensity intervals, they may end being a bit shorter.

Note that as of this writing (March 13, 2018), some Garmin Edge 520’s have been known to perform workouts a bit slower than prescribed.  This is due to an issue in the recent firmware releases.  Garmin is aware of this issue.