Why is $Cr^{2+}$ reducing and $Mn^{3+}$ oxidising when both have $d^{4}$ configuration?

(N/A) $Cr^{2+}$ is reducing because its configuration changes from $d^{4}$ to $d^{3}$,where the $d^{3}$ configuration has a stable half-filled $t_{2g}$ level.
On the other hand,the change from $Mn^{3+}$ to $Mn^{2+}$ results in the $d^{5}$ configuration,which is half-filled and possesses extra stability.