Why do the actual electron configurations for some elements differ from those assigned using the aufbau?

This is because not only the Aufbau's principle is obeyed in the electronic configuration. The other two rules are Hund's Rule and Pauli's Exclusion Principle.

The Aufbau's Principle says that the filling of the electron should start from the highest energy level to the lowest energy level. So, it would start with 1, to 2, and so on until 7. Hund's Rule states that each orbital of a subshell must be singly occupied before filling the two spaces for the electrons. Lastly, the Pauli's Exclusion Principle states that no two electrons in the subshell would have the same direction of spin. Together, these three rules/principles are followed such that no electron in the all of the elements would have same electronic configuration or quantum numbers.