Nernst equation: half cell vs overall reaction
It is coincidence, since 
$$\ce{AgCl -&gt; Ag+ + Cl-}$$
is not a half cell reaction.

The silver chloride electrode functions as a redox electrode and the reaction is between the silver metal (Ag) and its salt — silver chloride (AgCl). There are two ways to represent the processes (half reactions) happening on the electrode:
$$\ce{AgCl + e- -&gt; Ag + Cl-}$$
or
$$\ce{Ag+ + e- -&gt; Ag}$$
$$\ce{AgCl -&gt; Ag+ + Cl-}$$

And you should be applying Nernst equation to one of the two half cell reactions shown above.

The reason why you got the same result/answer is because the Nernst equation for silver chloride electrode has the dependence on the activity or effective concentration of only chloride-ions.

$$E = E^0 - \frac{\mathrm RT}{\mathrm F}\ln(a(\ce{Cl^-}))$$