Why do sequence of increasing energy of molecular orbital is different for some compounds?

The greater the overlap, the more the bonding orbital is lowered and the anti-bonding orbital is raised in energy relative to atomic orbitals. For e.g., the extent of overlapping in case of sigma – orbital is more than that in pi – orbital. Consequently, the energy of a ?2p_z is lower than the energy of bonding ?2p_x or ?2p_y MOs. 

So, basically it depends upon the extent of overlaping of orbitals.