Why can we disregard stoichiometric coefficients when computing reduction potentials?
You have to be careful of the difference between $E^\circ$ and $E$. $E^\circ$ is the standard electrode potential at defined conditions, some of which are mentioned in GODPARTICLE's answer. Standard thermodynamic conditions are usually


A temperature of 298 K
A pressure of gaseous components of 1 atm (or 1 bar)
A concentration of 1 M


The reference reduction potentials you quote are all standard potentials. These are the contributions to the cell potential when the half-cells meet the criteria above. 

$E$ is the cell potential at all other conditions. The correction of $E^\circ$ to $E$ involves temperature, pressure, concentration, and stoichiometry. Behold the Nernst equation:

$$E=E^\circ - \dfrac{RT}{nF}\ln{Q}$$


$R$ is the ideal gas constant (this is thermo after all)
$n$ is the number of electrons transferred - here is the stoichiometry!
$F$ is the Faraday constant 
$Q$ is the reaction quotient - which accounts for changes in pressure and concentration