The voltage given for any reduction is totally meaningless in the absence of either a comparison to another reduction potential or an oxidation potential. Absolute energies are totally meaningless. Only differences matter.
As usual then, we need references. For example, heat of formation of standard Gibbs energy if formation are set to 0 for elements in their standard forms at standard state.
Likewise, for reduction potentials, we need a reference, and that reference if the standard hydrogen electrode, whose standard reduction potential is 0 by fiat. All other potentials can be determined by comparison.