If you have any number of nucleons in a nucleus and you weigh each nucleon separately and then add their weights together, the total will be greater than if you weighed the nucleus all at once. Same number of particles--same particles, in fact. So how does this work? Once you figure out the answer (which is easier than it might seem), you get an idea of how things work and why what seems so common-sense on a macrophysical level makes no sense on a microphysical level and vice-versa.