why does the elements from calcium doesn't follow the 2n^2 rule?

as per this rule the 3rd shell can have 18 electrons, but the element take calcium its configuration is 2,8,8,2 . where as, we can accomodate 2 in the 3rd shell itself but why it is in 4th one?

According to Aufbau principle states, in the ground state of the atoms, the orbital are filled in increasing order of their energies.

That means electrons first occupy the lowest energy orbital available to them and enter in the higher energy orbital only after the lower energy orbitals are filled.

The order in which the electrons are filled is :

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f and so on

As per this principle, we can observe that, the energy of 3d orbital is more than the 4s orbital. Hence after fulfilling the 3p orbital the rest two electrons of Ca enter into 4s orbital instead of 3d orbital. Hence the electron configuration is 2,8,8,2 instead of 2,8,10.