The app converts all sets, reps, and weight schemes into a Projected 1 Rep Max (more info on Projected 1RM) number. So doing less reps or less weight will lower this number regardless of which one you reduce. The app will see you did less than recommended and will recommend slightly less next time. The opposite is true if you do more than recommended. If you are unable to complete the recommended sets, the app will see this and next time lower the prediction by a little bit.