The E^ values for Mn and Zn are more negative than expected because

The values of standard electrode potentials for Mn, and Zn are more negative than expected. The reason behind this is that they have exactly half-filled and fully-filled configuration ons. $$\begin{aligned} & { }_{25} \mathrm{Mn}: \left[\mathrm{Ar}\right] 3 d^5 4 s^2 \\ & { }_{30} \mathrm{Zn}:[\mathrm{Ar}] 3 d^{10} 4 s^2\end{aligned}$$