Notice that each of the metal ions has a different 'atmosphere' of charges around it. The cations in the metal are surrounded by electrons and other cations; the cations in the solution are surrounded mostly by water molecules (with other ions farther off). Atoms right on the boundary line will 'feel' different charges on the solution side and on the metal side. This is where the voltage comes from.
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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/redox/faq/print-why-electrode-voltages.shtml