Read about the mass defect.
m = P_S / c² * V_fohs
P_S and V_fohs do not exist in the standard model. The gravitational microcuviture of protons/neutrons cause changes in the P_S (static vacuum pressure) that therefor changes the mass. P_S is around 1.3 * 10^26 N/m².
This is from the Basic Structures of Matter - Supergravitation Unified Theory model, not the standard model.