The action of breathing is by its very nature extremely variable and difficult to control accurately.

When you first start training, you may find that your results vary widely between different training sessions.

As you get used to the action of breathing against resistance with maximum effort, you should find that your results become more consistent and controllable. You may still find that there is variation from day to day, depending on your physical condition and state of mind on a particular day, just as with any other form of exercise.