why E0M2+/M values are not regular for first row transition metals?

In general, the E^o values are supposed to reduce down the group (due to general increase in sum of first and second ionisation enthalpies). But the value of E​^o for Mn, Ni and Zn are more negative than expected from the trend. ​The stability of the half-filled d sub-shell in Mn²⁺ and the completely filled d_¹⁰ configuration in Zn²⁺ are related to their E^o values, whereas E^o for Ni is related to the highest negative H^o_hyd​.
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