Knotting is common in long polymers and significantly affects the polymer behavior. It is generally believed that knotting should increase polymer bending energy. However, many recent studies found that knotting can surprisingly decrease polymer bending energy, while the reason remains unclear. In this work, we quantitatively explained this surprising phenomenon using the tube model. In this model, polymer segments in a knot core are confined in a virtual tube. The tube affects the bending of knots in two opposite ways. First, the tube is curved to assume a knotted shape, which increases the bending. Second, the tube confinement suppresses the bending. We built a phase diagram to quantify the competition of these two effects. The second effect overwhelms under certain conditions because of two crucial yet often overlooked factors: (i) excluded volume interactions reduce the accessible diameter of the tube and (ii) polymer segments tend to escape the tube and thus produce an entropic force, which straightens the conformation. Overall, this work clarifies the special role of bending in polymer knots and demonstrates the usefulness of the tube model for polymer knots.