Why is zinc more reactive than copper? In terms of electronegativity, from what I understand electronegativity increases going across the period, so surely this should mean that zinc less readily loses its outer shell electrons than copper? Zinc has a greater nuclear charge but the outer shell electrons are in the same shell, so should the outer shell electrons in zinc not experience a stronger attraction to the nucleus? I have a feeling that this has something to do with which sub shells the electrons are removed from in copper and zinc...
You have to think about the whole process.  When a metal loses electrons to make a metal ion the following happens:


The metallic bonds holding the metal atoms together are broken.
The metal atom loses the electrons.
The resulting metal ion is hydrated.


In your analysis you are only focusing on step 2.  The enthalpy and entropy of the entire process factor into the reduction potential.