All the other eV values are relative to hydrogen, as far as I know. So, essentially hydrogen was picked as the standard (why exactly hydrogen, I don't know) and voltage produced when other elements/ions were oxidised/reduced by it was observed.
Hmm which one do I use bro? I'm trying to find the cell potential of the elements: Aluminium and bromine.
Hence I need the oxidation reaction of bromine - but it's got 2 voltages.