Cu has electronic configuration [Ar] 3d¹⁰4s¹. Formation of Cu²⁺ involves disturbing a stable 3d¹⁰ configuration. So, the reduced form of Cu²⁺ is more stable than the oxidized form of Cu. Therefore, the value of E° is positive for Cu.

For Zinc, the electronic configuration is [Ar]3d¹⁰4s². Removing two electrons gives a stable configuration [Ar]3d¹⁰ with completely filled orbitals. So, the oxidized form is more stable than the reduced form. Therefore, the value of E° is negative for Zn