There is unsymmetric filling of $\mathrm{e}_{\mathrm{g}}$ subset of $\mathrm{Cu}^2$ ion, while there is symmetrical distribution in $\mathrm{t}_{2 g}$ set, if the complex has same ligand there will be equal repulsion which leads to symmetrical bond length along $\mathrm{t}_{2 \mathrm{~g}}$ but due to uneven filling of electron in $\mathrm{e}_{\mathrm{g}}$ subset, either octahedral will be elongated or compressed.