The workouts in your first training weeks are designed to be appropriate for your initial fitness level. To improve, it is important to gradually increase the intensity and duration of your training sessions. That's why the first workouts may be shorter in duration.
As you progress and improve, the training plan will evolve to extend the duration of your workouts. There may also be times when it is beneficial to take a break and give yourself an easier week to recover after a few hard weeks of training. In these cases, you may receive workouts that are shorter than the availability you have indicated.