SMPS Input Circuit - Formulas resolving diodes, capacitors, voltage, etc
The AC wall voltage is 120V (rms).  But the actual peak value is sqrt(2)*120 = 169.7V.  

This doubled is 339V which is very close to the values you are seeing.  

I would expect your 120V rms wall voltage is really more like 124V rms.  This would give an actual peak voltage of 124*sqrt(2) = 175V.  Double that and you have your measured voltage (minus the diode drops).