What causes some atoms of the same element to have different atomic masses?

Elements have a certain number of protons(+), neutrons(0) and electrons(-). The atomic number shows the number of protons and electrons in the element (has to have the same number of each to stay stable). The atomic mass shows the number of neutrons. For example, Hydrogen has an atomic mass of ~1.01 (and an atomic number of 1). That means that it has 1 electron and 1 proton but no neutron (1 proton+0 neutrons=1). Helium has an atomic mass of ~4.00 (and an atomic number of 2). That means that it has 2 electrons, 2 protons and 2 neutrons (2 protons+2 neutrons=4).

Hope you can understand. :S