This is an effect of activity coefficients and ionic strength. Generally, at modestly increasing salt concentrations the activity coefficients decrease. I say modestly because this does not generally hold in brine solutions. By addiing additional salt to your solution you have increased the ionic strength. The pH is formally defined as the negative log of the hydrogen ion activity. The activity is calculated by mulitplying the activity coefficient "gamma" times the hydrogen ion concentration. Therefore increasing salt, increased the ionic strength, decreased the activity coefficient of the proton and your measured pH is therefore less. Of course you also run into other problems if you were measuring with a glass electrode, things like junction potentials etc but this is way beyond the scope of your question. For a further discussion of activity coefficients, ionic strength, I would suggest looking up the Davies equation for activity coefficients. These concepts are all easily google-able and available in Harris' text on Quantitative Chemical Analysis. Hope this was helpful.