This is because of the 2 isotopes of chlorine.
Isotopes are atoms with the same number of protons (in the case of chlorine that means 17 protons) but different numbers of neutrons. Chlorine has two common isotopes, Chlorine-35 and Chlorine-37.
The atomic mass, or as it is properly called, the RELATIVE atomic mass, is a weighted average of the masses of all the isotopes of chlorine. Relative Atomic Mass = (75/100 x 35) + (25/100 x 37) = 35.5