Why are there initially different pH values for the two solutions (NaCl and CH3COOH) used in the buffer solution experiment?

The difference in pH values for the two solutions is due to their chemical properties. NaCl is a neutral salt, so its solution will have a pH around 7. CH3COOH is a weak acid, so its solution will have a pH less than 7 due to the presence of hydrogen ions resulting from the partial dissociation of the acid.