Electron Magnetic Moment's Difference From Classical Mechanics

[Phyzwizz]

I was reading the Wikipedia article on Electron magnetic dipole moments and it mentioned that the "g-factor" is need in determining the magnetic moment of an electron because it varies by approximately two from the prediction of classical mechanics. Why exactly does this discrepancy occur?

 

[dextercioby]

Because the classical orbital motion is a totally different thing compared to the quantum mechanical angular momentum which has 2 distinctive parts: orbital (which has a classical counterpart) and spin (which doesn't have a classical counterpart).

It's the same difference as computing the deflection angle on star radiation due to solar gravitational field in Newtonian mechanics and gravity versus Einstein's GR.