The main reason for the diverting numbers (per 2 / per pair) is the fact that there are neutral endings that contain 5 and not 4 protons. These are an extra neutron positions that are optional, but often are essential (must have) when they support a opposing nuclet. Without the 5th for support the structure would be unstable and show a nuclear reaction to resettle.
Another reason for the diverting numbers is the fact that there are nuclets that have 9 protons which is an odd number again (Boron nuclet).
The Lithium nuclet (7 protons) is a morphed structure of 4 original tetrahedrons (helium nuclets) and this morphed Lithium is an odd number on its own. As a nuclet is has 6 and follows the pair wise growth numbering.