Table salt or sodium chloride with the formula NaCl has no acid or base properties whatsoever. When added to water, sodium chloride is neutral and the pH does not change. Table salt also has no buffering properties. A buffer is a salt solution that can resist change in pH when acid and base is added. Table salt is neutral. The glass was labeled pH 7.0 assuming the water was pH 7.0 to start and did not change when salt was added. It probably had a little carbonic acid in the juice.