The explanation is very long but i will try to make the shortest answer
Transition series belong to the f block of the periodic table.The effective nuclear charge(
This is the same reason why they are exceptions in ionization energy.Ionization energy partly depends on the size of atom, and the transition elements are exceptions in size.So, they are also the exceptions in ionization energy.
The exceptions in electronic configurations occur due to the stability of half and full filled orbitals.If you check the configurations of these exceptions, all the sub shells are either half filled or full filled which is not possible if we try to write the configurations according to Hund's rule.
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